Re: [PATCH] mm: thp: Set the accessed flag for old pages on accessfault.

From: Ni zhan Chen
Date: Tue Oct 16 2012 - 05:37:51 EST

On 10/01/2012 10:59 PM, Andrea Arcangeli wrote:
Hi Will,

On Mon, Oct 01, 2012 at 02:51:45PM +0100, Will Deacon wrote:
+void huge_pmd_set_accessed(struct mm_struct *mm, struct vm_area_struct *vma,
+ unsigned long address, pmd_t *pmd, pmd_t orig_pmd)
+ pmd_t entry;
+ spin_lock(&mm->page_table_lock);
+ entry = pmd_mkyoung(orig_pmd);
+ if (pmdp_set_access_flags(vma, address & HPAGE_PMD_MASK, pmd, entry, 0))
+ update_mmu_cache(vma, address, pmd);
If the pmd is being splitted, this may not be a trasnhuge pmd anymore
by the time you obtained the lock. (orig_pmd could be stale, and it
wasn't verified with pmd_same either)

Could you tell me when should call pmd_same in general?

The lock should be obtained through pmd_trans_huge_lock.

if (pmd_trans_huge_lock(orig_pmd, vma) == 1)
set young bit

On x86:

int pmdp_set_access_flags(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmdp,
pmd_t entry, int dirty)
int changed = !pmd_same(*pmdp, entry);


if (changed && dirty) {
*pmdp = entry;

with dirty == 0 it looks like it won't make any difference, but I
guess your arm pmdp_set_access_flag is different.

However it seems "dirty" means write access and so the invocation
would better match the pte case:

if (pmdp_set_access_flags(vma, address & HPAGE_PMD_MASK, pmd, entry,

But note, you still have to update it even when "dirty" == 0, or it'll
still infinite loop for read accesses.

+ spin_unlock(&mm->page_table_lock);
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..d5c007d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3537,7 +3537,11 @@ retry:
if (unlikely(ret & VM_FAULT_OOM))
goto retry;
return ret;
+ } else {
+ huge_pmd_set_accessed(mm, vma, address, pmd,
+ orig_pmd);
return 0;

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