Re: [PATCH] mm/userfaultfd: Fix release hang over concurrent GUP

[Andrew Morton]

On Wed, 12 Mar 2025 10:51:31 -0400 Peter Xu <peterx@xxxxxxxxxx> wrote:

> This patch should fix a possible userfaultfd release() hang during
> concurrent GUP.
> 
> This problem was initially reported by Dimitris Siakavaras in July 2023 [1]
> in a firecracker use case.  Firecracker has a separate process handling
> page faults remotely, and when the process releases the userfaultfd it can
> race with a concurrent GUP from KVM trying to fault in a guest page during
> the secondary MMU page fault process.
> 
> A similar problem was reported recently again by Jinjiang Tu in March 2025
> [2], even though the race happened this time with a mlockall() operation,
> which does GUP in a similar fashion.
> 
> In 2017, commit 656710a60e36 ("userfaultfd: non-cooperative: closing the
> uffd without triggering SIGBUS") was trying to fix this issue.  AFAIU, that
> fixes well the fault paths but may not work yet for GUP.  In GUP, the issue
> is NOPAGE will be almost treated the same as "page fault resolved" in
> faultin_page(), then the GUP will follow page again, seeing page missing,
> and it'll keep going into a live lock situation as reported.
> 
> This change makes core mm return RETRY instead of NOPAGE for both the GUP
> and fault paths, proactively releasing the mmap read lock.  This should
> guarantee the other release thread make progress on taking the write lock
> and avoid the live lock even for GUP.
> 
> When at it, rearrange the comments to make sure it's uptodate.

It would be good to have a Fixes: target but this bug seems to be so
old that a bare cc:stable should be OK?