[PATCH v4] mm: introduce reference pages
John Hubbard
jhubbard at nvidia.com
Tue Jun 29 00:19:22 PDT 2021
On 6/19/21 2:20 AM, Peter Collingbourne wrote:
> Introduce a new syscall, refpage_create, which returns a file
> descriptor which may be mapped using mmap. Such a mapping is similar
> to an anonymous mapping, but instead of clean pages being backed by the
> zero page, they are instead backed by a so-called reference page, whose
> contents are specified using an argument to refpage_create. Loads from
> the mapping will load directly from the reference page, and initial
> stores to the mapping will copy-on-write from the reference page.
Hi Peter,
Now that you have shown that this seems to have some performance
justification, I've taken a closer look at the patch, and have a handfull
of small suggestions, most of them very easy to deal with.
First of all: documentation of the new syscall. At the very least,
refpage.c could use a bunch of the wording that is in this patch's
commit description, at the top. I'm sure there are other places for new
syscall documentation (someone else probably knows where), but that would
be a good start.
>
> Reference pages are useful in circumstances where anonymous mappings
> combined with manual stores to memory would impose undesirable costs,
> either in terms of performance or RSS. Use cases are focused on heap
> allocators and include:
>
> - Pattern initialization for the heap. This is where malloc(3) gives
> you memory whose contents are filled with a non-zero pattern
> byte, in order to help detect and mitigate bugs involving use
> of uninitialized memory. Typically this is implemented by having
> the allocator memset the allocation with the pattern byte before
> returning it to the user, but for large allocations this can result
> in a significant increase in RSS, especially for allocations that
> are used sparsely. Even for dense allocations there is a needless
> impact to startup performance when it may be better to amortize it
> throughout the program. By creating allocations using a reference
> page filled with the pattern byte, we can avoid these costs.
As Kirill and Matthew mentioned in the other thread, it would be good
to pass in the pattern as part of the syscall, instead of deducing it
in prep_refpage_private_data(). I'll cover that more in the diffs area.
>
> - Pre-tagged heap memory. Memory tagging [1] is an upcoming ARMv8.5
> feature which allows for memory to be tagged in order to detect
> certain kinds of memory errors with low overhead. In order to set
> up an allocation to allow memory errors to be detected, the entire
> allocation needs to have the same tag. The issue here is similar to
> pattern initialization in the sense that large tagged allocations
> will be expensive if the tagging is done up front. The idea is that
> the allocator would create reference pages with each of the possible
> memory tags, and use those reference pages for the large allocations.
>
> This patch includes specific optimizations for these use cases in
> order to reduce memory traffic. If the reference page consists of a
> single repeating byte, the page is initialized using memset, and on
> arm64 if the reference page consists of a uniformly tagged zero page,
> the DC GZVA instruction is used to initialize the page.
>
> In order to measure the performance and RSS impact of reference pages,
> I used the following microbenchmark program, which is intended to
> compare an implementation of heap pattern initialization that uses
> memset to initialize the pages against an implementation that uses
> reference pages:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/mman.h>
> #include <unistd.h>
>
> constexpr unsigned char pattern_byte = 0xaa;
>
> #define PAGE_SIZE 4096
>
> _Alignas(PAGE_SIZE) static unsigned char pattern[PAGE_SIZE];
>
> int main(int argc, char **argv) {
> if (argc < 3)
> return 1;
> bool use_refpage = argc > 3;
> size_t mmap_size = atoi(argv[1]);
> size_t touch_size = atoi(argv[2]);
>
> int refpage_fd;
> if (use_refpage) {
> memset(pattern, pattern_byte, PAGE_SIZE);
> refpage_fd = syscall(448, pattern, 0);
> }
> for (unsigned i = 0; i != 1000; ++i) {
> char *p;
> if (use_refpage) {
> p = (char *)mmap(0, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE,
> refpage_fd, 0);
> } else {
> p = (char *)mmap(0, mmap_size, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> memset(p, pattern_byte, mmap_size);
> }
> for (unsigned j = 0; j < touch_size; j += PAGE_SIZE)
> p[j] = 0;
> munmap(p, mmap_size);
> }
> }
>
That sample code would be very nice to include in a documentation
section for documentation too, once we figure out the best place to put
it. If no one else recommends anything, then I'd start with
Documentation/mm/reference_pages.rst.
> On a DragonBoard 845c with the powersave governor, and taking the
> median of 10 runs for each measurement, I measured the following
> results for real time (s):
>
> touch_size/mmap_size memset refpages improvement (95% CI)
> 4096/4096000 3.962194 0.026726 98.8015% +/- 1.14684%
> 2048000/4096000 3.925309 1.48081 61.8271% +/- 1.11911%
> 4096000/4096000 3.986275 3.385003 15.1205% +/- 0.227235%
>
> And the following for max RSS (KiB):
>
> touch_size/mmap_size memset refpages improvement (95% CI)
> 4096/4096000 6656 3448 49.3815% +/- 1.30339%
> 2048000/4096000 6696 4580 31.7053% +/- 1.16411%
> 4096000/4096000 6716 6684 none
>
> So we see a large improvement for sparsely used allocations, and even
> a modest perf improvement for fully utilized allocations as a result
> of touching the pages one fewer time (with memset: once in the kernel
> and once in userspace; with refpages: just once in the kernel).
>
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> Link: [1] https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/enhancing-memory-safety
> ---
> v4:
> - rebased to linux-next
> - added arch hooks to support MTE tagged reference pages
> - added optimizations for pages with pattern byte as well as uniformly MTE-tagged pages
> - added helper functions to avoid open-coding the reference page detection
> - wrote a microbenchmark program and got new perf results for the commit message
>
> As an alternative to introducing this syscall, I considered using
> userfaultfd to implement reference pages. However, after having taken
> a detailed look at the interface, it does not seem suitable to be
> used in the context of a general purpose allocator. For example,
> UFFD_FEATURE_FORK support would be required in order to correctly
> support fork(2) in a process that uses the allocator (although POSIX
> does not guarantee support for allocating after fork, many allocators
> including Scudo support it, and nothing stops the forked process from
> page faulting pre-existing allocations after forking anyway), but
> UFFD_FEATURE_FORK has been restricted to root by commit 3c1c24d91ffd
> ("userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK"),
> making it unsuitable for use in an allocator. Furthermore, even if
> the interface issues are resolved, I suspect (but have not measured)
> that the cost of the multiple context switches between kernel and
> userspace would be too high to be used in an allocator anyway.
>
> arch/alpha/kernel/syscalls/syscall.tbl | 1 +
> arch/arm/tools/syscall.tbl | 1 +
> arch/arm64/include/asm/mman.h | 15 ++++
> arch/arm64/include/asm/mte.h | 9 +-
> arch/arm64/include/asm/page.h | 2 +-
> arch/arm64/include/asm/unistd.h | 2 +-
> arch/arm64/include/asm/unistd32.h | 2 +
> arch/arm64/kernel/mte.c | 24 +++++
> arch/arm64/lib/mte.S | 7 +-
> arch/arm64/mm/fault.c | 41 ++++++++-
> arch/ia64/kernel/syscalls/syscall.tbl | 1 +
> arch/m68k/kernel/syscalls/syscall.tbl | 1 +
> arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
> arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
> arch/parisc/kernel/syscalls/syscall.tbl | 1 +
> arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
> arch/s390/kernel/syscalls/syscall.tbl | 1 +
> arch/sh/kernel/syscalls/syscall.tbl | 1 +
> arch/sparc/kernel/syscalls/syscall.tbl | 1 +
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
> include/linux/gfp.h | 11 ++-
> include/linux/highmem.h | 2 +-
> include/linux/huge_mm.h | 7 ++
> include/linux/mm.h | 39 ++++++++
> include/linux/mman.h | 19 ++++
> include/linux/syscalls.h | 3 +
> include/uapi/asm-generic/unistd.h | 5 +-
> kernel/sys_ni.c | 1 +
> mm/Makefile | 4 +-
> mm/gup.c | 2 +-
> mm/kasan/hw_tags.c | 2 +-
> mm/memory.c | 47 +++++++---
> mm/migrate.c | 4 +-
> mm/page_alloc.c | 2 +-
> mm/refpage.c | 98 +++++++++++++++++++++
> 39 files changed, 330 insertions(+), 34 deletions(-)
> create mode 100644 mm/refpage.c
>
> diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
> index a17687ed4b51..494edc5ca61c 100644
> --- a/arch/alpha/kernel/syscalls/syscall.tbl
> +++ b/arch/alpha/kernel/syscalls/syscall.tbl
> @@ -486,3 +486,4 @@
> 554 common landlock_create_ruleset sys_landlock_create_ruleset
> 555 common landlock_add_rule sys_landlock_add_rule
> 556 common landlock_restrict_self sys_landlock_restrict_self
> +558 common refpage_create sys_refpage_create
> diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
> index c5df1179fc5d..8fd7045f46b9 100644
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -460,3 +460,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
> index e3e28f7daf62..5c0da3f76ec7 100644
> --- a/arch/arm64/include/asm/mman.h
> +++ b/arch/arm64/include/asm/mman.h
> @@ -84,4 +84,19 @@ static inline bool arch_validate_flags(unsigned long vm_flags)
> }
> #define arch_validate_flags(vm_flags) arch_validate_flags(vm_flags)
>
> +struct refpage_private_data;
> +
> +void arch_prep_refpage_private_data(struct refpage_private_data *priv);
> +#define arch_prep_refpage_private_data arch_prep_refpage_private_data
> +
> +static inline void arch_prep_refpage_vma(struct vm_area_struct *vma)
> +{
> + vma->vm_flags |= VM_MTE_ALLOWED;
> +}
> +#define arch_prep_refpage_vma arch_prep_refpage_vma
> +
> +void arch_copy_refpage(struct page *page, unsigned long addr,
> + struct vm_area_struct *vma);
> +#define arch_copy_refpage arch_copy_refpage
> +
> #endif /* ! __ASM_MMAN_H__ */
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 67bf259ae768..b513f83010c7 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -37,7 +37,7 @@ void mte_free_tag_storage(char *storage);
> /* track which pages have valid allocation tags */
> #define PG_mte_tagged PG_arch_2
>
> -void mte_zero_clear_page_tags(void *addr);
> +void mte_zero_set_page_tags(void *addr);
We should preserve the existing mte_zero_clear_page_tags(), and just
implement it in terms of the new, more general mte_zero_set_page_tags().
This is because: a) it will remove some diffs from this patch, and more
importantly, b) the concept of zeroing is still a distinct and useful
thing to have here.
> void mte_sync_tags(pte_t *ptep, pte_t pte);
> void mte_copy_page_tags(void *kto, const void *kfrom);
> void mte_thread_init_user(void);
> @@ -48,13 +48,14 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg);
> long get_mte_ctrl(struct task_struct *task);
> int mte_ptrace_copy_tags(struct task_struct *child, long request,
> unsigned long addr, unsigned long data);
> +u8 mte_check_tag_zero_page(struct page *userpage);
>
> #else /* CONFIG_ARM64_MTE */
>
> /* unused if !CONFIG_ARM64_MTE, silence the compiler */
> #define PG_mte_tagged 0
>
> -static inline void mte_zero_clear_page_tags(void *addr)
> +static inline void mte_zero_set_page_tags(void *addr)
> {
> }
> static inline void mte_sync_tags(pte_t *ptep, pte_t pte)
> @@ -89,6 +90,10 @@ static inline int mte_ptrace_copy_tags(struct task_struct *child,
> {
> return -EIO;
> }
> +static inline u8 mte_check_tag_zero_page(struct page *userpage)
> +{
> + return 0;
> +}
>
> #endif /* CONFIG_ARM64_MTE */
>
> diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> index 993a27ea6f54..234f48688b1a 100644
> --- a/arch/arm64/include/asm/page.h
> +++ b/arch/arm64/include/asm/page.h
> @@ -33,7 +33,7 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
> unsigned long vaddr);
> #define __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE_MOVABLE
>
> -void tag_clear_highpage(struct page *to);
> +void tag_set_highpage(struct page *to, unsigned long tag);
Same reasoning here: let's preserve tag_clear_highpage(), as well.
> #define __HAVE_ARCH_TAG_CLEAR_HIGHPAGE
>
> #define clear_user_page(page, vaddr, pg) clear_page(page)
> diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
> index 727bfc3be99b..3cb206aea3db 100644
> --- a/arch/arm64/include/asm/unistd.h
> +++ b/arch/arm64/include/asm/unistd.h
> @@ -38,7 +38,7 @@
> #define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
> #define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
>
> -#define __NR_compat_syscalls 447
> +#define __NR_compat_syscalls 449
> #endif
>
> #define __ARCH_WANT_SYS_CLONE
> diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
> index 99ffcafc736c..2a116aa17fe7 100644
> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -901,6 +901,8 @@ __SYSCALL(__NR_landlock_create_ruleset, sys_landlock_create_ruleset)
> __SYSCALL(__NR_landlock_add_rule, sys_landlock_add_rule)
> #define __NR_landlock_restrict_self 446
> __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
> +#define __NR_refpage_create 448
> +__SYSCALL(__NR_refpage_create, sys_refpage_create)
>
> /*
> * Please add new compat syscalls above this comment and update
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 125a10e413e9..6a79240d5a77 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -453,3 +453,27 @@ int mte_ptrace_copy_tags(struct task_struct *child, long request,
>
> return ret;
> }
> +
> +u8 mte_check_tag_zero_page(struct page *userpage)
> +{
> + char *userpage_addr = page_address(userpage);
> + u8 tag;
> + int i;
> +
> + if (!test_bit(PG_mte_tagged, &userpage->flags))
> + return 0;
> +
> + tag = mte_get_mem_tag(userpage_addr) & 0xF;
> + if (tag == 0)
> + return 0;
> +
> + for (i = 0; i != PAGE_SIZE; ++i)
> + if (userpage_addr[i] != 0)
> + return 0;
> +
> + for (i = MTE_GRANULE_SIZE; i != PAGE_SIZE; i += MTE_GRANULE_SIZE)
> + if ((mte_get_mem_tag(userpage_addr + i) & 0xF) != tag)
> + return 0;
> +
> + return tag;
> +}
> diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
> index e83643b3995f..45be436c97af 100644
> --- a/arch/arm64/lib/mte.S
> +++ b/arch/arm64/lib/mte.S
> @@ -37,24 +37,23 @@ SYM_FUNC_START(mte_clear_page_tags)
> SYM_FUNC_END(mte_clear_page_tags)
>
> /*
> - * Zero the page and tags at the same time
> + * Zero the page and set tags at the same time
> *
> * Parameters:
> * x0 - address to the beginning of the page
> */
> -SYM_FUNC_START(mte_zero_clear_page_tags)
> +SYM_FUNC_START(mte_zero_set_page_tags)
> mrs x1, dczid_el0
> and w1, w1, #0xf
> mov x2, #4
> lsl x1, x2, x1
> - and x0, x0, #(1 << MTE_TAG_SHIFT) - 1 // clear the tag
>
> 1: dc gzva, x0
> add x0, x0, x1
> tst x0, #(PAGE_SIZE - 1)
> b.ne 1b
> ret
> -SYM_FUNC_END(mte_zero_clear_page_tags)
> +SYM_FUNC_END(mte_zero_set_page_tags)
>
> /*
> * Copy the tags from the source page to the destination one
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 349c488765ca..36355758ffc7 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -25,6 +25,7 @@
> #include <linux/perf_event.h>
> #include <linux/preempt.h>
> #include <linux/hugetlb.h>
> +#include <linux/mman.h>
>
> #include <asm/acpi.h>
> #include <asm/bug.h>
> @@ -939,9 +940,45 @@ struct page *alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
> return alloc_page_vma(flags, vma, vaddr);
> }
>
> -void tag_clear_highpage(struct page *page)
> +void tag_set_highpage(struct page *page, unsigned long tag)
> {
> - mte_zero_clear_page_tags(page_address(page));
> + unsigned long addr = (unsigned long)page_address(page);
> +
> + addr &= ~MTE_TAG_MASK;
> + addr |= tag << MTE_TAG_SHIFT;
> + mte_zero_set_page_tags((void *)addr);
> page_kasan_tag_reset(page);
> set_bit(PG_mte_tagged, &page->flags);
> }
> +
> +#define REFPAGE_OPTZN_MTE_TAGGED REFPAGE_OPTZN_ARCH
I see what you're doing with the arch layer here, but there's no need to
accept the minor drawbacks (of having this #define hidden away near the
bottom of a .c file). Instead, let's just put this into the list in
mm.h, and call it what it is, rather than "arch".
> +
> +void arch_prep_refpage_private_data(struct refpage_private_data *priv)
> +{
> + if (system_supports_mte()) {
> + u8 tag;
> +
> + if (!test_and_set_bit(PG_mte_tagged, &priv->refpage->flags))
> + mte_clear_page_tags(page_address(priv->refpage));
> +
> + tag = mte_check_tag_zero_page(priv->refpage);
> + if (tag) {
> + priv->optzn_kind = REFPAGE_OPTZN_MTE_TAGGED;
> + priv->optzn_info = tag;
> + return;
> + }
> + }
> +
> + prep_refpage_private_data(priv);
> +}
> +
> +void arch_copy_refpage(struct page *page, unsigned long addr,
> + struct vm_area_struct *vma)
> +{
> + struct refpage_private_data *priv = vma->vm_private_data;
> +
> + if (priv->optzn_kind == REFPAGE_OPTZN_MTE_TAGGED)
> + tag_set_highpage(page, priv->optzn_info);
> + else
> + copy_refpage(page, addr, vma);
> +}
> diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
> index 6d07742c57b8..c2209d83f3c3 100644
> --- a/arch/ia64/kernel/syscalls/syscall.tbl
> +++ b/arch/ia64/kernel/syscalls/syscall.tbl
> @@ -367,3 +367,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
> index 541bc1b3a8f9..0360cf474a49 100644
> --- a/arch/m68k/kernel/syscalls/syscall.tbl
> +++ b/arch/m68k/kernel/syscalls/syscall.tbl
> @@ -446,3 +446,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
> index a176faca2927..de85d758e564 100644
> --- a/arch/microblaze/kernel/syscalls/syscall.tbl
> +++ b/arch/microblaze/kernel/syscalls/syscall.tbl
> @@ -452,3 +452,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
> index c2d2e19abea8..b07c7293d2a3 100644
> --- a/arch/mips/kernel/syscalls/syscall_n32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
> @@ -385,3 +385,4 @@
> 444 n32 landlock_create_ruleset sys_landlock_create_ruleset
> 445 n32 landlock_add_rule sys_landlock_add_rule
> 446 n32 landlock_restrict_self sys_landlock_restrict_self
> +448 n32 refpage_create sys_refpage_create
> diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
> index ac653d08b1ea..7ebabb99dd06 100644
> --- a/arch/mips/kernel/syscalls/syscall_n64.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
> @@ -361,3 +361,4 @@
> 444 n64 landlock_create_ruleset sys_landlock_create_ruleset
> 445 n64 landlock_add_rule sys_landlock_add_rule
> 446 n64 landlock_restrict_self sys_landlock_restrict_self
> +448 n64 refpage_create sys_refpage_create
> diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
> index 253f2cd70b6b..a51149ac101c 100644
> --- a/arch/mips/kernel/syscalls/syscall_o32.tbl
> +++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
> @@ -434,3 +434,4 @@
> 444 o32 landlock_create_ruleset sys_landlock_create_ruleset
> 445 o32 landlock_add_rule sys_landlock_add_rule
> 446 o32 landlock_restrict_self sys_landlock_restrict_self
> +448 o32 refpage_create sys_refpage_create
> diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
> index e26187b9ab87..385565864861 100644
> --- a/arch/parisc/kernel/syscalls/syscall.tbl
> +++ b/arch/parisc/kernel/syscalls/syscall.tbl
> @@ -444,3 +444,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
> index aef2a290e71a..95cdd9f7dc06 100644
> --- a/arch/powerpc/kernel/syscalls/syscall.tbl
> +++ b/arch/powerpc/kernel/syscalls/syscall.tbl
> @@ -526,3 +526,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
> index 64d51ab5a8b4..92ed1260ffd9 100644
> --- a/arch/s390/kernel/syscalls/syscall.tbl
> +++ b/arch/s390/kernel/syscalls/syscall.tbl
> @@ -449,3 +449,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create sys_refpage_create
> diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
> index e0a70be77d84..f9d198cc2541 100644
> --- a/arch/sh/kernel/syscalls/syscall.tbl
> +++ b/arch/sh/kernel/syscalls/syscall.tbl
> @@ -449,3 +449,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
> index 603f5a821502..83533aa49340 100644
> --- a/arch/sparc/kernel/syscalls/syscall.tbl
> +++ b/arch/sparc/kernel/syscalls/syscall.tbl
> @@ -492,3 +492,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index ce763a12311c..054c69e395b5 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -452,3 +452,4 @@
> 445 i386 landlock_add_rule sys_landlock_add_rule
> 446 i386 landlock_restrict_self sys_landlock_restrict_self
> 447 i386 memfd_secret sys_memfd_secret
> +448 i386 refpage_create sys_refpage_create
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index f6b57799c1ea..1f24f0b66cbd 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -369,6 +369,7 @@
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> 447 common memfd_secret sys_memfd_secret
> +448 common refpage_create sys_refpage_create
>
> #
> # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
> index 235d67d6ceb4..96c27fb404ca 100644
> --- a/arch/xtensa/kernel/syscalls/syscall.tbl
> +++ b/arch/xtensa/kernel/syscalls/syscall.tbl
> @@ -417,3 +417,4 @@
> 444 common landlock_create_ruleset sys_landlock_create_ruleset
> 445 common landlock_add_rule sys_landlock_add_rule
> 446 common landlock_restrict_self sys_landlock_restrict_self
> +448 common refpage_create sys_refpage_create
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 55b2ec1f965a..ae3c763eb9e9 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -55,8 +55,9 @@ struct vm_area_struct;
> #define ___GFP_ACCOUNT 0x400000u
> #define ___GFP_ZEROTAGS 0x800000u
> #define ___GFP_SKIP_KASAN_POISON 0x1000000u
> +#define ___GFP_NOZERO 0x2000000u
> #ifdef CONFIG_LOCKDEP
> -#define ___GFP_NOLOCKDEP 0x2000000u
> +#define ___GFP_NOLOCKDEP 0x4000000u
> #else
> #define ___GFP_NOLOCKDEP 0
> #endif
> @@ -238,18 +239,24 @@ struct vm_area_struct;
> * %__GFP_SKIP_KASAN_POISON returns a page which does not need to be poisoned
> * on deallocation. Typically used for userspace pages. Currently only has an
> * effect in HW tags mode.
> + *
> + * %__GFP_NOZERO disables any implicit zeroing of the page (e.g. as a result
> + * of init_on_alloc=on). This flag should only be used to address specific
> + * performance bottlenecks and only if the page is clearly being fully
> + * initialized following the allocation.
> */
> #define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN)
> #define __GFP_COMP ((__force gfp_t)___GFP_COMP)
> #define __GFP_ZERO ((__force gfp_t)___GFP_ZERO)
> #define __GFP_ZEROTAGS ((__force gfp_t)___GFP_ZEROTAGS)
> #define __GFP_SKIP_KASAN_POISON ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
> +#define __GFP_NOZERO ((__force gfp_t)___GFP_NOZERO)
>
> /* Disable lockdep for GFP context tracking */
> #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
>
> /* Room for N __GFP_FOO bits */
> -#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
> #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
>
> /**
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 93ba33c09d12..6c5076dd1e9b 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -187,7 +187,7 @@ static inline void clear_highpage(struct page *page)
>
> #ifndef __HAVE_ARCH_TAG_CLEAR_HIGHPAGE
>
> -static inline void tag_clear_highpage(struct page *page)
> +static inline void tag_set_highpage(struct page *page, unsigned long tag)
> {
> }
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index f123e15d966e..36ecfc391b46 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -127,6 +127,13 @@ static inline bool transhuge_vma_suitable(struct vm_area_struct *vma,
>
> if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
> return false;
> +
> + /*
> + * Transparent hugepages not currently supported for anonymous VMAs with
> + * reference pages
> + */
> + if (unlikely(is_refpage_vma(vma)))
> + return false;
> return true;
> }
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a127d93612fa..8cff9e0463b5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -32,6 +32,7 @@
> #include <linux/sched.h>
> #include <linux/pgtable.h>
> #include <linux/kasan.h>
> +#include <linux/fs.h>
>
> struct mempolicy;
> struct anon_vma;
> @@ -722,6 +723,42 @@ int vma_is_stack_for_current(struct vm_area_struct *vma);
> /* flush_tlb_range() takes a vma, not a mm, and can care about flags */
> #define TLB_FLUSH_VMA(mm,flags) { .vm_mm = (mm), .vm_flags = (flags) }
>
> +extern const struct file_operations refpage_file_operations;
> +
> +struct refpage_private_data {
> + struct page *refpage;
> + u8 optzn_kind;
How about:
u8 content_type;
> + u8 optzn_info;
and:
u8 pattern[16]; // or whatever size the enums go up to, see below
> +};
> +
> +#define REFPAGE_OPTZN_NONE 0
For this next set, how about How about REFPAGE_CONTENT_TYPE_ for a prefix?
The spelling of OPTZN is tough, and there's no particular need internally
to call these out as optimizations.
So then this one becomes:
#define REFPAGE_CONTENT_TYPE_USER_SET 0
> +#define REFPAGE_OPTZN_PATTERN 1
> +#define REFPAGE_OPTZN_ARCH 2
And for the last one, let's avoid the arch hiding and just call it what it
is, no reason not to:
#define REFPAGE_CONTENT_TYPE_MTE_TAGGED 2
> +
> +static inline bool is_refpage_vma(struct vm_area_struct *vma)
> +{
> + return vma->vm_file && vma->vm_file->f_op == &refpage_file_operations;
> +}
> +
> +static inline struct page *get_vma_refpage(struct vm_area_struct *vma)
> +{
> + struct refpage_private_data *priv = vma->vm_private_data;
> +
> + BUG_ON(!is_refpage_vma(vma));
> + return priv->refpage;
> +}
> +
> +static inline int is_refpage_pfn(struct vm_area_struct *vma, unsigned long pfn)
> +{
> + return is_refpage_vma(vma) && pfn == page_to_pfn(get_vma_refpage(vma));
> +}
> +
> +static inline int is_zero_or_refpage_pfn(struct vm_area_struct *vma,
> + unsigned long pfn)
> +{
> + return is_zero_pfn(pfn) || is_refpage_pfn(vma, pfn);
> +}
> +
I don't think this helper function is helping enough to justify itself,
seeing as how it is quite clear when the implementation is used instead. No
big deal either way, though.
> struct mmu_gather;
> struct inode;
>
> @@ -2977,6 +3014,8 @@ static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
> DECLARE_STATIC_KEY_MAYBE(CONFIG_INIT_ON_ALLOC_DEFAULT_ON, init_on_alloc);
> static inline bool want_init_on_alloc(gfp_t flags)
> {
> + if (flags & __GFP_NOZERO)
> + return false;
> if (static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
> &init_on_alloc))
> return true;
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index ebb09a964272..cdf8f8245c78 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -2,6 +2,7 @@
> #ifndef _LINUX_MMAN_H
> #define _LINUX_MMAN_H
>
> +#include <linux/fs.h>
> #include <linux/mm.h>
> #include <linux/percpu_counter.h>
>
> @@ -123,6 +124,24 @@ static inline bool arch_validate_flags(unsigned long flags)
> #define arch_validate_flags arch_validate_flags
> #endif
>
> +void prep_refpage_private_data(struct refpage_private_data *priv);
> +#ifndef arch_prep_refpage_private_data
> +#define arch_prep_refpage_private_data prep_refpage_private_data
> +#endif
> +
> +#ifndef arch_prep_refpage_vma
> +static inline void arch_prep_refpage_vma(struct vm_area_struct *vma)
> +{
> +}
> +#define arch_prep_refpage_vma arch_prep_refpage_vma
> +#endif
> +
> +void copy_refpage(struct page *page, unsigned long addr,
> + struct vm_area_struct *vma);
> +#ifndef arch_copy_refpage
> +#define arch_copy_refpage copy_refpage
> +#endif
> +
> /*
> * Optimisation macro. It is equivalent to:
> * (x & bit1) ? bit2 : 0
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 69c9a7010081..303a28a86500 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -864,6 +864,9 @@ asmlinkage long sys_mremap(unsigned long addr,
> unsigned long old_len, unsigned long new_len,
> unsigned long flags, unsigned long new_addr);
>
> +/* mm/refpage.c */
> +asmlinkage long sys_refpage_create(const void __user *content, unsigned long flags);
> +
> /* security/keys/keyctl.c */
> asmlinkage long sys_add_key(const char __user *_type,
> const char __user *_description,
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index a9d6fcd95f42..54cede7db5f0 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -878,8 +878,11 @@ __SYSCALL(__NR_landlock_restrict_self, sys_landlock_restrict_self)
> __SYSCALL(__NR_memfd_secret, sys_memfd_secret)
> #endif
>
> +#define __NR_refpage_create 448
> +__SYSCALL(__NR_refpage_create, sys_refpage_create)
> +
> #undef __NR_syscalls
> -#define __NR_syscalls 448
> +#define __NR_syscalls 449
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 30971b1dd4a9..bc65a54eb2a4 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -300,6 +300,7 @@ COND_SYSCALL(migrate_pages);
> COND_SYSCALL_COMPAT(migrate_pages);
> COND_SYSCALL(move_pages);
> COND_SYSCALL_COMPAT(move_pages);
> +COND_SYSCALL(refpage_create);
>
> COND_SYSCALL(perf_event_open);
> COND_SYSCALL(accept4);
> diff --git a/mm/Makefile b/mm/Makefile
> index e3436741d539..137adc22bf50 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -35,10 +35,10 @@ CFLAGS_init-mm.o += $(call cc-disable-warning, override-init)
> CFLAGS_init-mm.o += $(call cc-disable-warning, initializer-overrides)
>
> mmu-y := nommu.o
> -mmu-$(CONFIG_MMU) := highmem.o memory.o mincore.o \
> +mmu-$(CONFIG_MMU) := highmem.o ioremap.o memory.o mincore.o \
> mlock.o mmap.o mmu_gather.o mprotect.o mremap.o \
> msync.o page_vma_mapped.o pagewalk.o \
> - pgtable-generic.o rmap.o vmalloc.o ioremap.o
> + pgtable-generic.o refpage.o rmap.o vmalloc.o
>
>
> ifdef CONFIG_CROSS_MEMORY_ATTACH
> diff --git a/mm/gup.c b/mm/gup.c
> index 42b8b1fa6521..ba1b7bd7a0a0 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -548,7 +548,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> goto out;
> }
>
> - if (is_zero_pfn(pte_pfn(pte))) {
> + if (is_zero_or_refpage_pfn(vma, pte_pfn(pte))) {
> page = pte_page(pte);
> } else {
> ret = follow_pfn_pte(vma, address, ptep, flags);
> diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
> index ed5e5b833d61..3c433e430c80 100644
> --- a/mm/kasan/hw_tags.c
> +++ b/mm/kasan/hw_tags.c
> @@ -253,7 +253,7 @@ void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags)
> int i;
>
> for (i = 0; i != 1 << order; ++i)
> - tag_clear_highpage(page + i);
> + tag_set_highpage(page + i, 0);
Here, we could avoid this diff, by preserving tag_clear_highpage(). And
that's good, because the current diff is making the code just ever so
slightly worse. :)
> } else {
> kasan_unpoison_pages(page, order, init);
> }
> diff --git a/mm/memory.c b/mm/memory.c
> index db86558791f1..8b32bdd215b7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -614,7 +614,7 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> return vma->vm_ops->find_special_page(vma, addr);
> if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> return NULL;
> - if (is_zero_pfn(pfn))
> + if (is_zero_or_refpage_pfn(vma, pfn))
> return NULL;
> if (pte_devmap(pte))
> return NULL;
> @@ -640,7 +640,7 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
> }
> }
>
> - if (is_zero_pfn(pfn))
> + if (is_zero_or_refpage_pfn(vma, pfn))
> return NULL;
>
> check_pfn:
> @@ -2166,7 +2166,7 @@ static bool vm_mixed_ok(struct vm_area_struct *vma, pfn_t pfn)
> return true;
> if (pfn_t_special(pfn))
> return true;
> - if (is_zero_pfn(pfn_t_to_pfn(pfn)))
> + if (is_zero_or_refpage_pfn(vma, pfn_t_to_pfn(pfn)))
> return true;
> return false;
> }
> @@ -2990,22 +2990,29 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> pte_t entry;
> int page_copied = 0;
> struct mmu_notifier_range range;
> + unsigned long pfn;
>
> if (unlikely(anon_vma_prepare(vma)))
> goto oom;
>
> - if (is_zero_pfn(pte_pfn(vmf->orig_pte))) {
> + pfn = pte_pfn(vmf->orig_pte);
> + if (is_zero_pfn(pfn)) {
> new_page = alloc_zeroed_user_highpage_movable(vma,
> vmf->address);
> if (!new_page)
> goto oom;
> } else {
> - new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
> - vmf->address);
> + bool refpage = is_refpage_pfn(vma, pfn);
> +
> + new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE |
> + (refpage ? __GFP_NOZERO : 0),
> + vma, vmf->address);
> if (!new_page)
> goto oom;
>
> - if (!cow_user_page(new_page, old_page, vmf)) {
> + if (refpage) {
> + arch_copy_refpage(new_page, vmf->address, vma);
> + } else if (!cow_user_page(new_page, old_page, vmf)) {
> /*
> * COW failed, if the fault was solved by other,
> * it's fine. If not, userspace would re-fault on
> @@ -3739,11 +3746,16 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> if (unlikely(pmd_trans_unstable(vmf->pmd)))
> return 0;
>
> - /* Use the zero-page for reads */
> + /* Use the zero-page, or reference page if set, for reads */
> if (!(vmf->flags & FAULT_FLAG_WRITE) &&
> !mm_forbids_zeropage(vma->vm_mm)) {
> - entry = pte_mkspecial(pfn_pte(my_zero_pfn(vmf->address),
> - vma->vm_page_prot));
> + unsigned long pfn;
> +
> + if (unlikely(is_refpage_vma(vma)))
> + pfn = page_to_pfn(get_vma_refpage(vma));
> + else
> + pfn = my_zero_pfn(vmf->address);
> + entry = pte_mkspecial(pfn_pte(pfn, vma->vm_page_prot));
> vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> vmf->address, &vmf->ptl);
> if (!pte_none(*vmf->pte)) {
> @@ -3764,9 +3776,18 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> /* Allocate our own private page. */
> if (unlikely(anon_vma_prepare(vma)))
> goto oom;
> - page = alloc_zeroed_user_highpage_movable(vma, vmf->address);
> - if (!page)
> - goto oom;
> +
> + if (unlikely(is_refpage_vma(vma))) {
> + page = alloc_page_vma(GFP_HIGHUSER_MOVABLE | __GFP_NOZERO, vma,
> + vmf->address);
> + if (!page)
> + goto oom;
> + arch_copy_refpage(page, vmf->address, vma);
> + } else {
> + page = alloc_zeroed_user_highpage_movable(vma, vmf->address);
> + if (!page)
> + goto oom;
> + }
>
> if (mem_cgroup_charge(page, vma->vm_mm, GFP_KERNEL))
> goto oom_free_page;
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 23cbd9de030b..9a897676ff95 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2774,8 +2774,8 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
> pmd_t *pmdp;
> pte_t *ptep;
>
> - /* Only allow populating anonymous memory */
> - if (!vma_is_anonymous(vma))
> + /* Only allow populating anonymous memory without a reference page */
> + if (!vma_is_anonymous(vma) || is_refpage_vma(vma))
> goto abort;
>
> pgdp = pgd_offset(mm, addr);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8836e54721ae..6ca831c1821f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1283,7 +1283,7 @@ static void kernel_init_free_pages(struct page *page, int numpages, bool zero_ta
>
> if (zero_tags) {
> for (i = 0; i < numpages; i++)
> - tag_clear_highpage(page + i);
> + tag_set_highpage(page + i, 0);
> return;
> }
>
> diff --git a/mm/refpage.c b/mm/refpage.c
> new file mode 100644
> index 000000000000..ee95e281d2d4
> --- /dev/null
> +++ b/mm/refpage.c
> @@ -0,0 +1,98 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include <linux/anon_inodes.h>
> +#include <linux/fs_context.h>
> +#include <linux/highmem.h>
> +#include <linux/mman.h>
> +#include <linux/mount.h>
> +#include <linux/syscalls.h>
> +
> +void prep_refpage_private_data(struct refpage_private_data *priv)
> +{
> + u8 *addr = page_address(priv->refpage);
> + u8 pattern = addr[0];
> + int i;
> +
> + for (i = 1; i != PAGE_SIZE; ++i)
> + if (addr[i] != pattern)
> + return;
> +
> + priv->optzn_kind = REFPAGE_OPTZN_PATTERN;
> + priv->optzn_info = pattern;
> +}
> +
I am hoping that this doesn't remain in its current form, because of
the API discussions. Probably we'll end up with setting a pattern instead
of deducing it.
> +void copy_refpage(struct page *page, unsigned long addr,
> + struct vm_area_struct *vma)
> +{
> + struct refpage_private_data *priv = vma->vm_private_data;
> +
> + if (priv->optzn_kind == REFPAGE_OPTZN_PATTERN)
> + memset(page_address(page), priv->optzn_info, PAGE_SIZE);
> + else
> + copy_user_highpage(page, priv->refpage, addr, vma);
> +}
> +
> +static void put_refpage_private_data(struct refpage_private_data *priv)
Can you please rename this to free_refpage_private_data()? It's a little more
accurate.
> +{
> + put_page(priv->refpage);
> + kfree(priv);
> +}
> +
> +static int refpage_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + vma_set_anonymous(vma);
> + vma->vm_private_data = vma->vm_file->private_data;
> + arch_prep_refpage_vma(vma);
> + return 0;
> +}
> +
> +static int refpage_release(struct inode *inode, struct file *file)
> +{
> + put_refpage_private_data(file->private_data);
> + return 0;
> +}
> +
> +const struct file_operations refpage_file_operations = {
> + .mmap = refpage_mmap,
> + .release = refpage_release,
> +};
> +
> +SYSCALL_DEFINE2(refpage_create, const void *__user, content, unsigned long,
> + flags)
From the API discussion (and using a simpler syntax to illustrate this), it
seems like the following would be close:
enum content_type {
BYTE_PATTERN,
FOUR_BYTE_PATTERN,
...
FULL_4KB_PAGE
};
int refpage_create(const void *__user content, enum content_type, unsigned long flags);
...and if content_type == BYTE_PATTERN, then content is a pointer to just one byte of
data, and so forth for the other enum values.
> +{
> + unsigned long content_addr = (unsigned long)content;
> + struct page *userpage;
> + struct refpage_private_data *private_data;
> + int fd;
> +
> + if (flags != 0)
> + return -EINVAL;
> +
> + if ((content_addr & (PAGE_SIZE - 1)) != 0 ||
> + get_user_pages(content_addr, 1, 0, &userpage, 0) != 1)
> + return -EFAULT;
> +
> + private_data = kzalloc(sizeof(struct refpage_private_data), GFP_KERNEL);
> + if (!private_data) {
> + put_page(userpage);
> + return -ENOMEM;
> + }
> +
> + private_data->refpage = alloc_page(GFP_KERNEL);
> + if (!private_data->refpage) {
> + kfree(private_data);
> + put_page(userpage);
> + return -ENOMEM;
> + }
> +
> + copy_highpage(private_data->refpage, userpage);
> + arch_prep_refpage_private_data(private_data);
> + put_page(userpage);
> +
> + fd = anon_inode_getfd("[refpage]", &refpage_file_operations,
> + private_data, O_RDONLY | O_CLOEXEC);
> + if (fd < 0)
> + put_refpage_private_data(private_data);
And here, a couple of things:
1) I think there's a bug in the fd < 0 case, because you're only freeing
one of the two pages (there's an alloc_page() call, and a gup call above).
2) It's jarring to have part the error handling in three different ways:
returning -EFAULT directly, coding each error case to undo the growing
set of operations, and finally, jumping out to another routine here for
fd < 0.
Even for a small routine, that's too error-prone. Instead, one of the
following will be cleaner and safer too:
a) use goto and labels to unwind, or
b) use a no-fail cleanup routine to unwind
and either way, do it for all cases (or at least all of them after the first
trivial -EFAULT return.
> +
> + return fd;
> +}
>
thanks,
--
John Hubbard
NVIDIA
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