Re: [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance

[Lorenzo Stoakes <ljs@kernel.org>]

On Wed, May 20, 2026 at 02:39:49PM -0700, Yang Shi wrote:
> On Wed, May 20, 2026 at 3:34 AM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
> >
> > On 5/19/26 14:53, Lorenzo Stoakes wrote:
> > > On Mon, May 18, 2026 at 12:56:59PM -0700, Suren Baghdasaryan wrote:
> > >
> > >>>
> > >>> I think we either need to fix `fork()`, or keep the current
> > >>> behavior of dropping the VMA lock before performing I/O.
> > >>
> > >> I see. So, this problem arises from the fact that we are changing the
> > >> pagefaults requiring I/O operation to hold VMA lock...
> > >> And you want to lock VMA on fork only if vma_is_anonymous(vma) ||
> > >> is_cow_mapping(vma->vm_flags). So, we will be blocking page faults for
> > >> anonymous and COW VMAs only while holding mmap_write_lock, preventing
> > >> any VMA modification. On the surface, that looks ok to me but I might
> > >> be missing some corner cases. If nobody sees any obvious issues, I
> > >> think it's worth a try.
> > >
> > > Not sure if you noticed but I did raise concerns ;)
> > >
> > > I wonder if you've confused the fault path and fork here, as I think Barry has
> > > been a little unclear on that.
> > >
> > > What's being suggested in this thread is to fundamentally change fork behaviour
> > > so it's different from the entire history of the kernel (or - presumably - at
> > > least recent history :)
> > I don't want fork() to become different in that regard.
> >
> > There is already a slight difference with vs. without per-VMA locks, because
> > there is a window in-between us taking the write mmap_lock and all the per-VMA
> > locks. I raised that previously [1] and assumed that it is probably fine.
> >
> > I also raised in the past why I think we must not allow concurrent page faults,
> > at least as soon as anonymous memory is involved [2].
>
> Thanks for sharing the context, it is quite helpful to understand the
> race conditions. Because Lorenzo also raised the concern about page
> fault race, I will reply to all the concerns regarding page fault race
> together in this thread.
>
> IIUC, there is already some sort of race with per vma lock. Before per
> vma lock, mmap_lock did lock everything. So page fault happened either
> before fork or after fork. But page fault can happen on other VMAs
> which have not been lock'ed yet during fork with per vma lock. For
> example, we have 3 VMAs, we lock the first VMA, but page fault still
> can happen on the other 2 VMAs during fork if they already have
> anon_vma. This is the status quo now, but it seems not harmful.
>
> The bad race shared by David is caused by racing with copy page. So it
> seems like it will be fine as long as we serialize copy page against
> page fault if I don't miss anything. Since we decide whether to copy
> page or not by checking vma->anon_vma, so it seems fine to not take
> vma lock if vma->anon_vma is NULL. This will not introduce more race
> either because setting up a new  anon_vma in page fault or madvise
> requires taking mmap_lock according to the earlier discussions.

NAK. No.

We're not doing this, we're not changing how fork fundamentally behaves because
of concerns about the fault path.

I've delineated exactly why I think this is a problem and you're pressing ahead
without addressing those concerns.

So at this point I'm going to be a grumpy maintainer and just say no, stop
please :)

Let's fix this in the right place. You don't fix a leak in the roof by repairing
a shelf next door :)

Thanks, Lorenzo


>
> Thanks,
> Yang
>
> >
> > ... and I raised that this is pretty much slower by design right now: "Well, the
> > design decision that CONFIG_PER_VMA_LOCK made for now to make page faults fast
> > and to make blocking any page faults from happening to  be slower ..." [3]
> >
> > [1] https://lore.kernel.org/all/970295ab-e85d-7af3-76e6-df53a5c52f8b@redhat.com/
> > [2] https://lore.kernel.org/all/7e3f35cc-59b9-bf12-b8b1-4ed78223844a@redhat.com/
> > [3] https://lore.kernel.org/all/2efa2c89-3765-721d-2c3c-00590054aa5b@redhat.com/
> >
> > --
> > Cheers,
> >
> > David
> >