linux-kernel - [PATCH v3] mm: thp: Set the accessed flag for old pages on access fault.

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Date:	Thu, 25 Oct 2012 17:44:31 +0100
From:	Will Deacon <will.deacon@....com>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	mhocko@...e.cz, peterz@...radead.org, akpm@...ux-foundation.org,
	Will Deacon <will.deacon@....com>,
	Chris Metcalf <cmetcalf@...era.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH v3] mm: thp: Set the accessed flag for old pages on access fault.

On x86 memory accesses to pages without the ACCESSED flag set result in the
ACCESSED flag being set automatically. With the ARM architecture a page access
fault is raised instead (and it will continue to be raised until the ACCESSED
flag is set for the appropriate PTE/PMD).

For normal memory pages, handle_pte_fault will call pte_mkyoung (effectively
setting the ACCESSED flag). For transparent huge pages, pmd_mkyoung will only
be called for a write fault.

This patch ensures that faults on transparent hugepages which do not result
in a CoW update the access flags for the faulting pmd.

Cc: Chris Metcalf <cmetcalf@...era.com>
Cc: Kirill A. Shutemov <kirill@...temov.name>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---

Ok chaps, I rebased this thing onto today's next (which basically
necessitated a rewrite) so I've reluctantly dropped my acks and kindly
ask if you could eyeball the new code, especially where the locking is
concerned. In the numa code (do_huge_pmd_prot_none), Peter checks again
that the page is not splitting, but I can't see why that is required.

Cheers,

Will

 include/linux/huge_mm.h |    4 ++++
 mm/huge_memory.c        |   22 ++++++++++++++++++++++
 mm/memory.c             |    7 ++++++-
 3 files changed, 32 insertions(+), 1 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 4f0f948..766fb27 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -8,6 +8,10 @@ extern int do_huge_pmd_anonymous_page(struct mm_struct *mm,
 extern int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 			 pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
 			 struct vm_area_struct *vma);
+extern void huge_pmd_set_accessed(struct mm_struct *mm,
+				  struct vm_area_struct *vma,
+				  unsigned long address, pmd_t *pmd,
+				  pmd_t orig_pmd, int dirty);
 extern int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			       unsigned long address, pmd_t *pmd,
 			       pmd_t orig_pmd);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3c14a96..f024d98 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -932,6 +932,28 @@ out:
 	return ret;
 }
 
+void huge_pmd_set_accessed(struct mm_struct *mm,
+			   struct vm_area_struct *vma,
+			   unsigned long address,
+			   pmd_t *pmd, pmd_t orig_pmd,
+			   int dirty)
+{
+	pmd_t entry;
+	unsigned long haddr;
+
+	spin_lock(&mm->page_table_lock);
+	if (unlikely(!pmd_same(*pmd, orig_pmd)))
+		goto unlock;
+
+	entry = pmd_mkyoung(orig_pmd);
+	haddr = address & HPAGE_PMD_MASK;
+	if (pmdp_set_access_flags(vma, haddr, pmd, entry, dirty))
+		update_mmu_cache_pmd(vma, address, pmd);
+
+unlock:
+	spin_unlock(&mm->page_table_lock);
+}
+
 static int do_huge_pmd_wp_page_fallback(struct mm_struct *mm,
 					struct vm_area_struct *vma,
 					unsigned long address,
diff --git a/mm/memory.c b/mm/memory.c
index f21ac1c..bcbc084 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3650,12 +3650,14 @@ retry:
 
 		barrier();
 		if (pmd_trans_huge(orig_pmd) && !pmd_trans_splitting(orig_pmd)) {
+			unsigned int dirty = flags & FAULT_FLAG_WRITE;
+
 			if (pmd_numa(vma, orig_pmd)) {
 				do_huge_pmd_numa_page(mm, vma, address, pmd,
 						      flags, orig_pmd);
 			}
 
-			if ((flags & FAULT_FLAG_WRITE) && !pmd_write(orig_pmd)) {
+			if (dirty && !pmd_write(orig_pmd)) {
 				ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
 							  orig_pmd);
 				/*
@@ -3665,6 +3667,9 @@ retry:
 				 */
 				if (unlikely(ret & VM_FAULT_OOM))
 					goto retry;
+			} else {
+				huge_pmd_set_accessed(mm, vma, address, pmd,
+						      orig_pmd, dirty);
 			}
 
 			return ret;
-- 
1.7.4.1

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