Re: [PATCH v4] mm: userfaultfd: fix race of userfaultfd_move and swap cache

[Andrew Morton]

On Wed, 11 Jun 2025 13:16:25 +0800 Kairui Song <ryncsn@xxxxxxxxx> wrote:

> This commit fixes two kinds of races, they may have different results:
> 
> Barry reported a BUG_ON in commit c50f8e6053b0, we may see the same
> BUG_ON if the filemap lookup returned NULL and folio is added to swap
> cache after that.
> 
> If another kind of race is triggered (folio changed after lookup) we
> may see RSS counter is corrupted:
> 
> [  406.893936] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0
> type:MM_ANONPAGES val:-1
> [  406.894071] BUG: Bad rss-counter state mm:ffff0000c5a9ddc0
> type:MM_SHMEMPAGES val:1
> 
> Because the folio is being accounted to the wrong VMA.
> 
> I'm not sure if there will be any data corruption though, seems no.
> The issues above are critical already.

Thanks, I pasted this into the patch's changelog.