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

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

[Will Deacon]

From: Steve Capper <steve.capper@arm.com>

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@redhat.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---

Hello again,

This is another fix for an issue that we discovered when porting THP to
ARM but it somehow managed to slip through the cracks.

Will

 include/linux/huge_mm.h |    3 +++
 mm/huge_memory.c        |   12 ++++++++++++
 mm/memory.c             |    4 ++++
 3 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 4c59b11..bbc62ad 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -8,6 +8,9 @@ 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);
 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..ee9cc3b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -914,6 +914,18 @@ out_free_pages:
 	goto out;
 }
 
+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);
+	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;
 		}
 	}
-- 
1.7.4.1

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

[Kirill A. Shutemov]

On Mon, Oct 01, 2012 at 02:51:45PM +0100, Will Deacon wrote:
> 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);

I think putting it to 'else' is wrong. You should not touch pmd, if it's
under splitting.

>  			}
> +
>  			return 0;
>  		}
>  	}

-- 
 Kirill A. Shutemov

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

[Andrea Arcangeli]

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)

The lock should be obtained through pmd_trans_huge_lock.

  if (pmd_trans_huge_lock(orig_pmd, vma) == 1)
  {
	set young bit
	spin_unlock(&mm->page_table_lock);
  }


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);

	VM_BUG_ON(address & ~HPAGE_PMD_MASK);

	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,
	    flags & FAULT_FLAG_WRITE))


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;

Thanks,
Andrea

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

[Will Deacon]

On Mon, Oct 01, 2012 at 03:59:44PM +0100, Andrea Arcangeli wrote:
> Hi Will,

Hi Andrea, Kirill,

Thanks for the comments.

> 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)
> 
> The lock should be obtained through pmd_trans_huge_lock.
> 
>   if (pmd_trans_huge_lock(orig_pmd, vma) == 1)
>   {
> 	set young bit
> 	spin_unlock(&mm->page_table_lock);
>   }

I didn't notice that -- thanks. I'll move the locking outside of the
_set_accessed function and direct it via that function instead.

> 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);
> 
> 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> 
> 	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.

We use the generic code, which ignores the dirty argument. Still, we should
pass the correct value through anyway, so I'll fix that too.

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

Yup. v2 to follow once we've re-run our testing.

Cheers,

Will

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

[Ni zhan Chen]

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
> 	spin_unlock(&mm->page_table_lock);
>    }
>
>
> 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);
>
> 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> 	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,
> 	    flags & FAULT_FLAG_WRITE))
>
>
> 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;
> Thanks,
> Andrea
>
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