Re: [PATCH] mm/khugepaged: Check again on anon uffd-wp during isolation

[David Hildenbrand]

On 05.04.23 17:51, Peter Xu wrote:
> Khugepaged collapse an anonymous thp in two rounds of scans.  The 2nd round
> done in __collapse_huge_page_isolate() after hpage_collapse_scan_pmd(),
> during which all the locks will be released temporarily. It means the
> pgtable can change during this phase before 2nd round starts.
>
> It's logically possible some ptes got wr-protected during this phase, and
> we can errornously collapse a thp without noticing some ptes are
> wr-protected by userfault.  e1e267c7928f wanted to avoid it but it only did
> that for the 1st phase, not the 2nd phase.
>
> Since __collapse_huge_page_isolate() happens after a round of small page
> swapins, we don't need to worry on any !present ptes - if it existed
> khugepaged will already bail out.  So we only need to check present ptes
> with uffd-wp bit set there.
>
> This is something I found only but never had a reproducer, I thought it was
> one caused a bug in Muhammad's recent pagemap new ioctl work, but it turns
> out it's not the cause of that but an userspace bug.  However this seems to
> still be a real bug even with a very small race window, still worth to have
> it fixed and copy stable.
>
> Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> Fixes: e1e267c7928f ("khugepaged: skip collapse if uffd-wp detected")
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>   mm/khugepaged.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a19aa140fd52..42ac93b4bd87 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -575,6 +575,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>   			result = SCAN_PTE_NON_PRESENT;
>   			goto out;
>   		}
> +		if (pte_uffd_wp(pteval)) {
> +			result = SCAN_PTE_UFFD_WP;
> +			goto out;
> +		}
>   		page = vm_normal_page(vma, address, pteval);
>   		if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
>   			result = SCAN_PAGE_NULL;

Yes, I agree that would be a small race where it could happen.

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb