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

[Matthew Wilcox]

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.

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.