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

[Ni zhan Chen <nizhan.chen@gmail.com>]

On 10/26/2012 12:44 AM, Will Deacon wrote:
> 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.

Could you write changlog?

>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Will Deacon <will.deacon@arm.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

Could you explain why you not call pmd_trans_huge_lock to confirm the 
pmd is splitting or stable as Andrea point out?

>
>   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;