[PATCH 1/2] mm: make faultaround produce old ptes

[Vinayak Menon]

Based on Kirill's patch [1].

Currently, faultaround code produces young pte.  This can screw up
vmscan behaviour[2], as it makes vmscan think that these pages are hot
and not push them out on first round.

During sparse file access faultaround gets more pages mapped and all of
them are young.  Under memory pressure, this makes vmscan swap out anon
pages instead, or to drop other page cache pages which otherwise stay
resident.

Modify faultaround to produce old ptes, so they can easily be reclaimed
under memory pressure.

This can to some extend defeat the purpose of faultaround on machines
without hardware accessed bit as it will not help us with reducing the
number of minor page faults.

Making the faultaround ptes old results in a unixbench regression for some
architectures [3][4]. But on some architectures it is not found to cause
any regression. So by default produce young ptes and provide an option for
architectures to make the ptes old.

[1] http://lkml.kernel.org/r/1463488366-47723-1-git-send-email-kirill.shutemov@linux.intel.com
[2] https://lkml.kernel.org/r/1460992636-711-1-git-send-email-vinmenon@codeaurora.org
[3] https://marc.info/?l=linux-kernel&m=146582237922378&w=2
[4] https://marc.info/?l=linux-mm&m=146589376909424&w=2

Signed-off-by: Vinayak Menon <vinmenon at codeaurora.org>
---
 include/linux/mm-arch-hooks.h | 7 +++++++
 include/linux/mm.h            | 2 ++
 mm/filemap.c                  | 4 ++++
 mm/memory.c                   | 5 +++++
 4 files changed, 18 insertions(+)

diff --git a/include/linux/mm-arch-hooks.h b/include/linux/mm-arch-hooks.h
index 4efc3f56..0322b98 100644
--- a/include/linux/mm-arch-hooks.h
+++ b/include/linux/mm-arch-hooks.h
@@ -22,4 +22,11 @@ static inline void arch_remap(struct mm_struct *mm,
 #define arch_remap arch_remap
 #endif
 
+#ifndef arch_faultaround_pte_mkold
+static inline void arch_faultaround_pte_mkold(struct vm_fault *vmf)
+{
+}
+#define arch_faultaround_pte_mkold arch_faultaround_pte_mkold
+#endif
+
 #endif /* _LINUX_MM_ARCH_HOOKS_H */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7661156..be689a0 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -302,6 +302,7 @@ extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
 #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
 #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
 #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
+#define FAULT_FLAG_MKOLD	0x200	/* Make faultaround ptes old */
 
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
@@ -330,6 +331,7 @@ struct vm_fault {
 	gfp_t gfp_mask;			/* gfp mask to be used for allocations */
 	pgoff_t pgoff;			/* Logical page offset based on vma */
 	unsigned long address;		/* Faulting virtual address */
+	unsigned long fault_address;    /* Saved faulting virtual address */
 	pmd_t *pmd;			/* Pointer to pmd entry matching
 					 * the 'address' */
 	pud_t *pud;			/* Pointer to pud entry matching
diff --git a/mm/filemap.c b/mm/filemap.c
index 693f622..63c7bf4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -36,6 +36,7 @@
 #include <linux/cleancache.h>
 #include <linux/shmem_fs.h>
 #include <linux/rmap.h>
+#include <linux/mm-arch-hooks.h>
 #include "internal.h"
 
 #define CREATE_TRACE_POINTS
@@ -2677,6 +2678,9 @@ void filemap_map_pages(struct vm_fault *vmf,
 		if (vmf->pte)
 			vmf->pte += iter.index - last_pgoff;
 		last_pgoff = iter.index;
+
+		arch_faultaround_pte_mkold(vmf);
+
 		if (alloc_set_pte(vmf, NULL, page))
 			goto unlock;
 		unlock_page(page);
diff --git a/mm/memory.c b/mm/memory.c
index 24e9e1d..210dea3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3398,6 +3398,10 @@ int alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 	entry = mk_pte(page, vma->vm_page_prot);
 	if (write)
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+
+	if (vmf->flags & FAULT_FLAG_MKOLD)
+		entry = pte_mkold(entry);
+
 	/* copy-on-write page */
 	if (write && !(vma->vm_flags & VM_SHARED)) {
 		inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
@@ -3527,6 +3531,7 @@ static int do_fault_around(struct vm_fault *vmf)
 	pgoff_t end_pgoff;
 	int off, ret = 0;
 
+	vmf->fault_address = address;
 	nr_pages = READ_ONCE(fault_around_bytes) >> PAGE_SHIFT;
 	mask = ~(nr_pages * PAGE_SIZE - 1) & PAGE_MASK;
 
-- 
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