linux-kernel - [PATCH v2] mm: thp: Set the accessed flag for old pages on access fault.

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Message-Id: <1349197151-19645-1-git-send-email-will.deacon@arm.com>
Date:	Tue,  2 Oct 2012 17:59:11 +0100
From:	Will Deacon <will.deacon@....com>
To:	linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	akpm@...ux-foundation.org, mhocko@...e.cz, kirill@...temov.name,
	Will Deacon <will.deacon@....com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Steve Capper <steve.capper@....com>
Subject: [PATCH v2] 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: Andrea Arcangeli <aarcange@...hat.com>
Cc: Chris Metcalf <cmetcalf@...era.com>
Signed-off-by: Steve Capper <steve.capper@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
---

v2: - Use pmd_trans_huge_lock to guard against splitting pmds
    - Propogate dirty (write) flag to low-level pmd modifier

 include/linux/huge_mm.h |    2 ++
 mm/huge_memory.c        |    8 ++++++++
 mm/memory.c             |    9 ++++++++-
 3 files changed, 18 insertions(+), 1 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 4c59b11..5eb9b06 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -8,6 +8,8 @@ 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 vm_area_struct *vma,
+				  unsigned long address, pmd_t *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 d684934..1de3f9b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -914,6 +914,14 @@ out_free_pages:
 	goto out;
 }
 
+void huge_pmd_set_accessed(struct vm_area_struct *vma, unsigned long address,
+			   pmd_t *pmd, int dirty)
+{
+	pmd_t entry = pmd_mkyoung(*pmd);
+	if (pmdp_set_access_flags(vma, address & HPAGE_PMD_MASK, pmd, entry, dirty))
+		update_mmu_cache(vma, address, pmd);
+}
+
 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/memory.c b/mm/memory.c
index 5736170..f12f859 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3524,7 +3524,8 @@ retry:
 
 		barrier();
 		if (pmd_trans_huge(orig_pmd)) {
-			if (flags & FAULT_FLAG_WRITE &&
+			int dirty = flags & FAULT_FLAG_WRITE;
+			if (dirty &&
 			    !pmd_write(orig_pmd) &&
 			    !pmd_trans_splitting(orig_pmd)) {
 				ret = do_huge_pmd_wp_page(mm, vma, address, pmd,
@@ -3537,7 +3538,13 @@ retry:
 				if (unlikely(ret & VM_FAULT_OOM))
 					goto retry;
 				return ret;
+			} else if (pmd_trans_huge_lock(pmd, vma) == 1) {
+				if (likely(pmd_same(*pmd, orig_pmd)))
+					huge_pmd_set_accessed(vma, address, pmd,
+							      dirty);
+				spin_unlock(&mm->page_table_lock);
 			}
+
 			return 0;
 		}
 	}
-- 
1.7.4.1

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