[PATCH v2 1/3] arm64: KVM: Fix stage-2 PGD allocation to have per-page refcounting
Christoffer Dall
christoffer.dall at linaro.org
Wed Mar 11 07:25:05 PDT 2015
On Wed, Mar 11, 2015 at 3:03 PM, Andrew Jones <drjones at redhat.com> wrote:
> On Wed, Mar 11, 2015 at 02:20:56PM +0100, Christoffer Dall wrote:
>> On Wed, Mar 11, 2015 at 2:13 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
>> > Hi Christoffer,
>> >
>> > On 11/03/15 12:16, Christoffer Dall wrote:
>> >> Hi Marc,
>> >>
>> >> On Tue, Mar 10, 2015 at 07:06:59PM +0000, Marc Zyngier wrote:
>> >>> We're using __get_free_pages with to allocate the guest's stage-2
>> >>> PGD. The standard behaviour of this function is to return a set of
>> >>> pages where only the head page has a valid refcount.
>> >>>
>> >>> This behaviour gets us into trouble when we're trying to increment
>> >>> the refcount on a non-head page:
>> >>>
>> >>> page:ffff7c00cfb693c0 count:0 mapcount:0 mapping: (null) index:0x0
>> >>> flags: 0x4000000000000000()
>> >>> page dumped because: VM_BUG_ON_PAGE((*({ __attribute__((unused)) typeof((&page->_count)->counter) __var = ( typeof((&page->_count)->counter)) 0; (volatile typeof((&page->_count)->counter) *)&((&page->_count)->counter); })) <= 0)
>> >>> BUG: failure at include/linux/mm.h:548/get_page()!
>> >>> Kernel panic - not syncing: BUG!
>> >>> CPU: 1 PID: 1695 Comm: kvm-vcpu-0 Not tainted 4.0.0-rc1+ #3825
>> >>> Hardware name: APM X-Gene Mustang board (DT)
>> >>> Call trace:
>> >>> [<ffff80000008a09c>] dump_backtrace+0x0/0x13c
>> >>> [<ffff80000008a1e8>] show_stack+0x10/0x1c
>> >>> [<ffff800000691da8>] dump_stack+0x74/0x94
>> >>> [<ffff800000690d78>] panic+0x100/0x240
>> >>> [<ffff8000000a0bc4>] stage2_get_pmd+0x17c/0x2bc
>> >>> [<ffff8000000a1dc4>] kvm_handle_guest_abort+0x4b4/0x6b0
>> >>> [<ffff8000000a420c>] handle_exit+0x58/0x180
>> >>> [<ffff80000009e7a4>] kvm_arch_vcpu_ioctl_run+0x114/0x45c
>> >>> [<ffff800000099df4>] kvm_vcpu_ioctl+0x2e0/0x754
>> >>> [<ffff8000001c0a18>] do_vfs_ioctl+0x424/0x5c8
>> >>> [<ffff8000001c0bfc>] SyS_ioctl+0x40/0x78
>> >>> CPU0: stopping
>> >>>
>> >>> A possible approach for this is to split the compound page using
>> >>> split_page() at allocation time, and change the teardown path to
>> >>> free one page at a time.
>> >>>
>> >>> While we're at it, the PGD allocation code is reworked to reduce
>> >>> duplication.
>> >>>
>> >>> This has been tested on an X-Gene platform with a 4kB/48bit-VA host
>> >>> kernel, and kvmtool hacked to place memory in the second page of
>> >>> the hardware PGD (PUD for the host kernel). Also regression-tested
>> >>> on a Cubietruck (Cortex-A7).
>> >>>
>> >>> Reported-by: Mark Rutland <mark.rutland at arm.com>
>> >>> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
>> >>> ---
>> >>> arch/arm/include/asm/kvm_mmu.h | 9 ++---
>> >>> arch/arm/kvm/mmu.c | 77 +++++++++++++++++++++++++++++++---------
>> >>> arch/arm64/include/asm/kvm_mmu.h | 46 +++---------------------
>> >>> 3 files changed, 66 insertions(+), 66 deletions(-)
>> >>>
>> >>> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
>> >>> index 0187606..ff56f91 100644
>> >>> --- a/arch/arm/include/asm/kvm_mmu.h
>> >>> +++ b/arch/arm/include/asm/kvm_mmu.h
>> >>> @@ -162,18 +162,13 @@ static inline bool kvm_page_empty(void *ptr)
>> >>>
>> >>> #define KVM_PREALLOC_LEVEL 0
>> >>>
>> >>> -static inline int kvm_prealloc_hwpgd(struct kvm *kvm, pgd_t *pgd)
>> >>> -{
>> >>> - return 0;
>> >>> -}
>> >>> -
>> >>> -static inline void kvm_free_hwpgd(struct kvm *kvm) { }
>> >>> -
>> >>> static inline void *kvm_get_hwpgd(struct kvm *kvm)
>> >>> {
>> >>> return kvm->arch.pgd;
>> >>> }
>> >>>
>> >>> +static inline unsigned int kvm_get_hwpgd_order(void) { return S2_PGD_ORDER; }
>> >>> +
>> >>> struct kvm;
>> >>>
>> >>> #define kvm_flush_dcache_to_poc(a,l) __cpuc_flush_dcache_area((a), (l))
>> >>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> >>> index 69c2b4c..0a5457c 100644
>> >>> --- a/arch/arm/kvm/mmu.c
>> >>> +++ b/arch/arm/kvm/mmu.c
>> >>> @@ -634,6 +634,31 @@ int create_hyp_io_mappings(void *from, void *to, phys_addr_t phys_addr)
>> >>> __phys_to_pfn(phys_addr), PAGE_HYP_DEVICE);
>> >>> }
>> >>>
>> >>> +/* Free the HW pgd, one page at a time */
>> >>> +static void kvm_free_hwpgd(unsigned long hwpgd)
>> >>> +{
>> >>> + int i;
>> >>> +
>> >>> + for (i = 0; i < (1 << kvm_get_hwpgd_order()); i += PAGE_SIZE)
>> >>> + free_page(hwpgd + i);
>> >>> +}
>> >>> +
>> >>> +/* Allocate the HW PGD, making sure that each page gets its own refcount */
>> >>> +static int kvm_alloc_hwpgd(unsigned long *hwpgdp)
>> >>
>> >> I think this can be simplified somewhat by just returning an unsigned
>> >> long that can be 0 in the error case which will make the caller look
>> >> a little nicer too.
>> >
>> > Sure.
>> >
>> >>> +{
>> >>> + unsigned long hwpgd;
>> >>> + unsigned int order = kvm_get_hwpgd_order();
>> >>> +
>> >>> + hwpgd = __get_free_pages(GFP_KERNEL | __GFP_ZERO, order);
>> >>> + if (!hwpgd)
>> >>> + return -ENOMEM;
>> >>> +
>> >>> + split_page(virt_to_page((void *)hwpgd), order);
>> >>
>> >> nit: alloc_pages_exact() and free_pages_exact() seem to do this for us,
>> >> is it worth using those instead?
>> >
>> > Ah, I didn't know about these.
>> >
>> >> It would look something like this on top of your changes:
>> >>
>> >>
>> >> arch/arm/include/asm/kvm_mmu.h | 5 ++++-
>> >> arch/arm/kvm/mmu.c | 32 ++++++++++----------------------
>> >> arch/arm64/include/asm/kvm_mmu.h | 6 +++---
>> >> 3 files changed, 17 insertions(+), 26 deletions(-)
>> >>
>> >> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
>> >> index fc05ba8..4cf48c3 100644
>> >> --- a/arch/arm/include/asm/kvm_mmu.h
>> >> +++ b/arch/arm/include/asm/kvm_mmu.h
>> >> @@ -168,7 +168,10 @@ static inline void *kvm_get_hwpgd(struct kvm *kvm)
>> >> return kvm->arch.pgd;
>> >> }
>> >>
>> >> -static inline unsigned int kvm_get_hwpgd_order(void) { return S2_PGD_ORDER; }
>> >> +static inline unsigned int kvm_get_hwpgd_size(void)
>> >> +{
>> >> + return PTRS_PER_S2_PGD * sizeof(pgd_t);
>> >> +}
>> >>
>> >> struct kvm;
>> >>
>> >> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>> >> index 8e91bea3..5656d79 100644
>> >> --- a/arch/arm/kvm/mmu.c
>> >> +++ b/arch/arm/kvm/mmu.c
>> >> @@ -633,28 +633,17 @@ int create_hyp_io_mappings(void *from, void *to, phys_addr_t phys_addr)
>> >> }
>> >>
>> >> /* Free the HW pgd, one page at a time */
>> >> -static void kvm_free_hwpgd(unsigned long hwpgd)
>> >> +static void kvm_free_hwpgd(void *hwpgd)
>> >> {
>> >> - int i;
>> >> -
>> >> - for (i = 0; i < (1 << kvm_get_hwpgd_order()); i += PAGE_SIZE)
>> >> - free_page(hwpgd + i);
>> >> + free_pages_exact(hwpgd, kvm_get_hwpgd_size());
>> >> }
>> >>
>> >> /* Allocate the HW PGD, making sure that each page gets its own refcount */
>> >> -static int kvm_alloc_hwpgd(unsigned long *hwpgdp)
>> >> +static void *kvm_alloc_hwpgd(void)
>> >> {
>> >> - unsigned long hwpgd;
>> >> - unsigned int order = kvm_get_hwpgd_order();
>> >> -
>> >> - hwpgd = __get_free_pages(GFP_KERNEL | __GFP_ZERO, order);
>> >> - if (!hwpgd)
>> >> - return -ENOMEM;
>> >> -
>> >> - split_page(virt_to_page((void *)hwpgd), order);
>> >> + unsigned int size = kvm_get_hwpgd_size();
>> >>
>> >> - *hwpgdp = hwpgd;
>> >> - return 0;
>> >> + return alloc_pages_exact(size, GFP_KERNEL | __GFP_ZERO);
>> >> }
>> >>
>> >> /**
>> >> @@ -670,18 +659,17 @@ static int kvm_alloc_hwpgd(unsigned long *hwpgdp)
>> >> */
>> >> int kvm_alloc_stage2_pgd(struct kvm *kvm)
>> >> {
>> >> - int ret;
>> >> pgd_t *pgd;
>> >> - unsigned long hwpgd;
>> >> + void *hwpgd;
>> >>
>> >> if (kvm->arch.pgd != NULL) {
>> >> kvm_err("kvm_arch already initialized?\n");
>> >> return -EINVAL;
>> >> }
>> >>
>> >> - ret = kvm_alloc_hwpgd(&hwpgd);
>> >> - if (ret)
>> >> - return ret;
>> >> + hwpgd = kvm_alloc_hwpgd();
>> >> + if (!hwpgd)
>> >> + return -ENOMEM;
>> >>
>> >> /* When the kernel uses more levels of page tables than the
>> >> * guest, we allocate a fake PGD and pre-populate it to point
>> >> @@ -829,7 +817,7 @@ void kvm_free_stage2_pgd(struct kvm *kvm)
>> >> return;
>> >>
>> >> unmap_stage2_range(kvm, 0, KVM_PHYS_SIZE);
>> >> - kvm_free_hwpgd((unsigned long)kvm_get_hwpgd(kvm));
>> >> + kvm_free_hwpgd(kvm_get_hwpgd(kvm));
>> >> if (KVM_PREALLOC_LEVEL > 0)
>> >> kfree(kvm->arch.pgd);
>> >>
>> >> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
>> >> index 3668110..bbfb600 100644
>> >> --- a/arch/arm64/include/asm/kvm_mmu.h
>> >> +++ b/arch/arm64/include/asm/kvm_mmu.h
>> >> @@ -189,11 +189,11 @@ static inline void *kvm_get_hwpgd(struct kvm *kvm)
>> >> return pmd_offset(pud, 0);
>> >> }
>> >>
>> >> -static inline unsigned int kvm_get_hwpgd_order(void)
>> >> +static inline unsigned int kvm_get_hwpgd_size(void)
>> >> {
>> >> if (KVM_PREALLOC_LEVEL > 0)
>> >> - return PTRS_PER_S2_PGD_SHIFT;
>> >> - return S2_PGD_ORDER;
>> >> + return PTRS_PER_S2_PGD * PAGE_SIZE;
>> >> + return PTRS_PER_S2_PGD * sizeof(pgd_t);
>> >> }
>> >>
>> >> static inline bool kvm_page_empty(void *ptr)
>> >
>> > Looks good to me. I'll fold that into the patch, amend the commit
>> > message and resend.
>> >
>>
>> I've done that here:
>> https://git.linaro.org/people/christoffer.dall/linux-kvm-arm.git/shortlog/refs/heads/pgd-fixes-alt
>>
>> (except for the commit message, which I can also adjust).
>>
>> I gave this a quick spin on Juno and TC2 (but couldn't test 64K pages
>> because I don't have a platform for that handy this very second)...
>
> Tested with 64K config. Works for me (test was just a guest boot)
>
Thanks,
applied.
-Christoffer
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