Re: [PATCH mm-unstable] Revert "mm/khugepaged: remove redundant transhuge_vma_suitable() check"

[Yang Shi <shy828301@gmail.com>]

On Wed, Jul 20, 2022 at 4:13 AM Zach O'Keefe <zokeefe@google.com> wrote:
>
> A pmd should not cross a VMA boundary, which is normally enforced by
> vma_adjust_trans_huge(), and assumed by e.g. __split_huge_pmd_locked().
>
> In this regard, the transhuge_vma_suitable() check in
> hugepage_vma_check() is not redundant with the transhuge_vma_suitable()
> check previously in hugepage_vma_revalidate().
>
> The former validates the VMA itself, and checks that *some* memory
> in the VMA is suitable to collapse while the latter validates if
> collapsing at a specific address is suitable.  By removing the check on
> the faulting address, we've inadvertently allowed collapse of a pmd that
> can cross vma->vm_end. Revert this change.

Aha, yeah, nice catch.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Fixes: 143776e7512e ("mm/khugepaged: remove redundant transhuge_vma_suitable() check")
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> ---
> Apologies, Andrew. I think you've put the series description into this
> first patch (thank you).  Do you mind moving it into the next patch in the
> series,
> "mm: khugepaged: don't carry huge page to the next loop for !CONFIG_NUMA"?
> Note that the "mm: userspace hugepage collapse, v7" series doesn't actually
> depend on this patch, it was just a cleanup (and thus perhaps I shouldn't have
> included it in the series in the first place).
> ---
>  mm/khugepaged.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 2db6d0dd2981..69990dacde14 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -855,6 +855,8 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
>         if (!vma)
>                 return SCAN_VMA_NULL;
>
> +       if (!transhuge_vma_suitable(vma, address))
> +               return SCAN_ADDRESS_RANGE;
>         if (!hugepage_vma_check(vma, vma->vm_flags, false, false,
>                                 cc->is_khugepaged))
>                 return SCAN_VMA_CHECK;
> --
> 2.37.0.170.g444d1eabd0-goog
>