Re: [PATCH] mm: write protect MADV_FREE pages

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 Jan 2017 15:15:52 -0800 Shaohua Li <shli@fb.com> 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.

I'd like to see here a complete description of the bug's effects: waht
sort of workload will trigger it, what the end-user visible effects
are, etc.

> --- 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);

Is this the right way round?  There's still a window where we won't get
that write fault on the cleaned pte.  Should the pmd_wrprotect() happen
before the pmd_mkclean()?


>  		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);

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>