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

[Suren Baghdasaryan]

On Thu, Nov 27, 2025 at 2:29 PM Barry Song <21cnbao at gmail.com> wrote:
>
> On Fri, Nov 28, 2025 at 3:43 AM Matthew Wilcox <willy at 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 at 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.

Another corner case we need to consider is when there is a large VMA
covering most of the address space, so holding a VMA lock during IO
would resemble holding an mmap_lock, leading to the same issue that we
faced before "drop mmap locks for I/O". We discussed this with Matthew
in the context of the problem he mentioned (the page is reclaimed
before page fault retry happens) with no conclusion yet.

>
> >
> > > 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.

I don't know if this direction is the right one but I agree with
Matthew that we should consider alternatives before adopting a new
direction. Hopefully we can find one fix for both problems rather than
fixing each one in isolation.

>
> Thanks
> Barry
>