Re: [RFC PATCH 0/2] mm: continue using per-VMA lock when retrying page faults after I/O

[Barry Song <21cnbao@gmail.com>]

On Fri, Nov 28, 2025 at 3:43 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> [dropping individuals, leaving only mailing lists.  please don't send
> this kind of thing to so many people in future]
>
> On Thu, Nov 27, 2025 at 12:22:16PM +0800, Barry Song wrote:
> > On Thu, Nov 27, 2025 at 12:09 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Thu, Nov 27, 2025 at 09:14:36AM +0800, Barry Song wrote:
> > > > There is no need to always fall back to mmap_lock if the per-VMA
> > > > lock was released only to wait for pagecache or swapcache to
> > > > become ready.
> > >
> > > Something I've been wondering about is removing all the "drop the MM
> > > locks while we wait for I/O" gunk.  It's a nice amount of code removed:
> >
> > I think the point is that page fault handlers should avoid holding the VMA
> > lock or mmap_lock for too long while waiting for I/O. Otherwise, those
> > writers and readers will be stuck for a while.
>
> There's a usecase some of us have been discussing off-list for a few
> weeks that our current strategy pessimises.  It's a process with
> thousands (maybe tens of thousands) of threads.  It has much more mapped
> files than it has memory that cgroups will allow it to use.  So on a
> page fault, we drop the vma lock, allocate a page of ram, kick off the
> read, sleep waiting for the folio to come uptodate, once it is return,
> expecting the page to still be there when we reenter filemap_fault.
> But it's under so much memory pressure that it's already been reclaimed
> by the time we get back to it.  So all the threads just batter the
> storage re-reading data.

Is this entirely the fault of re-entering the page fault? Under extreme
memory pressure, even if we map the pages, they can still be reclaimed
quickly?

>
> If we don't drop the vma lock, we can insert the pages in the page table
> and return, maybe getting some work done before this thread is
> descheduled.

If we need to protect the page from being reclaimed too early, the fix
should reside within LRU management, not in page fault handling.

Also, I gave an example where we may not drop the VMA lock if the folio is
already up to date. That likely corresponds to waiting for the PTE mapping to
complete.

>
> This use case also manages to get utterly hung-up trying to do reclaim
> today with the mmap_lock held.  SO it manifests somewhat similarly to
> your problem (everybody ends up blocked on mmap_lock) but it has a
> rather different root cause.
>
> > I agree there’s room for improvement, but merely removing the "drop the MM
> > locks while waiting for I/O" code is unlikely to improve performance.
>
> I'm not sure it'd hurt performance.  The "drop mmap locks for I/O" code
> was written before the VMA locking code was written.  I don't know that
> it's actually helping these days.

I am concerned that other write paths may still need to modify the VMA, for
example during splitting. Tail latency has long been a significant issue for
Android users, and we have observed it even with folio_lock, which has much
finer granularity than the VMA lock.

>
> > The change would be much more complex, so I’d prefer to land the current
> > patchset first. At least this way, we avoid falling back to mmap_lock and
> > causing contention or priority inversion, with minimal changes.
>
> Uh, this is an RFC patchset.  I'm giving you my comment, which is that I
> don't think this is the right direction to go in.  Any talk of "landing"
> these patches is extremely premature.

While I agree that there are other approaches worth exploring, I
remain entirely unconvinced that this patchset is the wrong
direction. With the current retry logic, it substantially reduces
mmap_lock acquisitions and represents a clear low-hanging fruit.

Also, I am not referring to landing the RFC itself, but to a subsequent formal
patchset that retries using the per-VMA lock.

Thanks
Barry