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

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Date:	Mon, 1 Oct 2012 16:59:44 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org,
	mhocko@...e.cz, Steve Capper <steve.capper@....com>,
	Chris Metcalf <cmetcalf@...era.com>
Subject: Re: [PATCH] mm: thp: Set the accessed flag for old pages on access
 fault.

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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