Re: [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem re-acquired

[Ebru Akagunduz]

On Wed, Jun 15, 2016 at 11:06:10PM +0300, Kirill A. Shutemov wrote:
> Vlastimil noted[1] that pmd can be no longer valid after we drop
> mmap_sem. We need recheck it once mmap_sem taken again.
> 
> [1] http://lkml.kernel.org/r/12918dcd-a695-c6f4-e06f-69141c5f357f@xxxxxxx
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/huge_memory.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d7ccc8558187..0efdad975659 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2437,6 +2437,9 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>  			/* vma is no longer available, don't continue to swapin */
>  			if (hugepage_vma_revalidate(mm, address))
>  				return false;
> +			/* check if the pmd is still valid */
> +			if (mm_find_pmd(mm, address) != pmd)
> +				return false;
>  		}
Thanks for fixing this.

>  		if (ret & VM_FAULT_ERROR) {
>  			trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
> @@ -2522,6 +2525,9 @@ static void collapse_huge_page(struct mm_struct *mm,
>  	result = hugepage_vma_revalidate(mm, address);
>  	if (result)
>  		goto out;
> +	/* check if the pmd is still valid */
> +	if (mm_find_pmd(mm, address) != pmd)
> +		goto out;
>  
However here, I don't know do we need to check pmd.
Because in collapse_huge_page; pmd is newly created,
after taking mmap_sem read:
{

	pmd_t *pmd, _pmd;
        pte_t *pte;
        ...
        down_read(&mm->mmap_sem);
        result = hugepage_vma_revalidate(mm, address);
        ...
        pmd = mm_find_pmd(mm, address);
        ...

Therefore it did not seem like a problem for me.

>  	anon_vma_lock_write(vma->anon_vma);
>  
> -- 
> 2.8.1
>