Re: [PATCH] mm: write protect MADV_FREE pages

[Minchan Kim <minchan@kernel.org>]

Hi Shaohua,

On Mon, Jan 23, 2017 at 03:15:52PM -0800, Shaohua Li wrote:
> The page reclaim has an assumption writting to a page with clean pte
> should trigger a page fault, because there is a window between pte zero
> and tlb flush where a new write could come. If the new write doesn't
> trigger page fault, page reclaim will not notice it and think the page
> is clean and reclaim it. The MADV_FREE pages don't comply with the rule
> and the pte is just cleaned without writeprotect, so there will be no
> pagefault for new write. This will cause data corruption.

It's hard to understand.
Could you show me exact scenario seqence you have in mind?

Thanks.

> 
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: stable@kernel.org
> Signed-off-by: Shaohua Li <shli@fb.com>
> ---
>  mm/huge_memory.c | 1 +
>  mm/madvise.c     | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9a6bd6c..9cc5de5 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1381,6 +1381,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  			tlb->fullmm);
>  		orig_pmd = pmd_mkold(orig_pmd);
>  		orig_pmd = pmd_mkclean(orig_pmd);
> +		orig_pmd = pmd_wrprotect(orig_pmd);
>  
>  		set_pmd_at(mm, addr, pmd, orig_pmd);
>  		tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 0e3828e..bfb6800 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -373,6 +373,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  
>  			ptent = pte_mkold(ptent);
>  			ptent = pte_mkclean(ptent);
> +			ptent = pte_wrprotect(ptent);
>  			set_pte_at(mm, addr, pte, ptent);
>  			if (PageActive(page))
>  				deactivate_page(page);
> -- 
> 2.9.3
> 
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