[PATCH v6] mm: retry page faults once under the per-VMA lock

[Barry Song]

On Mon, Sep 28, 2026 at 6:55 AM Matthew Wilcox <willy at infradead.org> wrote:
>
> On Tue, Sep 22, 2026 at 09:55:23AM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Sep 21, 2026 at 07:57:23PM +0100, Matthew Wilcox wrote:
> > > more to say on this in two weeks at Plumbers.
> >
> > I look forward to it :)
>
> So while doing my slides, I realised that what we need to avoid doing
> is (a) sleeping while holding the mmap_lock (b) returning RETRY while
> holding the VMA lock
>
> And that turns out to be as simple as this patch:
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dd09c438fa23..94ed2333f8d8 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -723,6 +723,8 @@ enum {
>   */
>  static inline bool fault_flag_allow_retry_first(enum fault_flag flags)
>  {
> +       if (flags & FAULT_FLAG_VMA_LOCK)
> +               return false;
>         return (flags & FAULT_FLAG_ALLOW_RETRY) &&
>             (!(flags & FAULT_FLAG_TRIED));
>  }
>
> OK, this is a hack.  The function is spectacularly badly named, and
> needs to be renamed before a patch can go upstream.  But this should
> fix the contention on mmap_lock.

Thanks for your suggestion.
This is exactly what we did in Android Common Kernel before we had
Lorenzo's proposal (bypassing `fault_flag_allow_retry_first()`):

https://android.googlesource.com/kernel/common/+/1b9b045a586245cc1c29b2747c6586234c7f5bad%5E%21/#F2

At that time, we couldn't modify ACK with a GKI hook that introduced
a new flag. All previous versions of this patchset introduced new
flags, which would have broken the KMI.
So we had to choose between two evils:
1. mmap_lock contention
2. VMA lock contention
We chose the lesser evil — no. 2 — and sent the Android hook for
merging into ACK.

Now that we have Lorenzo's proposal, which doesn't require any new
flag, we are adding a new Android hook just to mimic what Lorenzo's
proposal already does:
https://android-review.googlesource.com/c/kernel/common/+/4307957/3/arch/arm64/mm/fault.c

Note that Lorenzo's proposal avoids mmap_lock contention without
introducing any new VMA lock contention. It also doesn't require a new
flag that would break KMI. So this is clearly the preferred approach.

>
> Could somebody try it?  I've verified it boots and runs some userspace
> fine, but I don't have the workload to test the contention.

Both Nanzhe and Hongru tested it before and reported the fork issue.

Best Regards
Barry