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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, May 19, 2026 at 12:53 PM Lorenzo Stoakes <ljs@kernel.org> 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 ;)

Sorry, I didn't realize your first comment was a conceptual objection
to this approach of allowing page faults to race with the fork.


>
> 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 :) and permit concurrent page faults to occur on a forking
> process.
>
> I absolutely object to this for being pretty crazy. I mean I'm not sure we
> really want to be simultaneously modifying page tables while invoking
> copy_page_range()? No?
>
> OK you cover anon and MAP_PRIVATE file-backed but hang on there's
> VM_COPY_ON_FORK too.. so PFN mapped, mixed map and (the accursed) UFFD W/P as
> well as possibly-guard region containing VMAs now can have page tables raced.

Ugh, yeah, I realize now this is a minefield. Resolving all possible
races there would not be trivial and might introduce other performance
issues.

>
> That's not to mention anything else that relies on serialisation here (this
> would be changing how forking has been done in general) that we may or may not
> know about.
>
> The risk level is high, for what amounts to a hack to work around the fault
> issue.
>
> I suggest that if we have a problem with the fault path, let's look at the fault
> path :)
>
> So yeah I'm very opposed to this unless I'm somehow horribly mistaken here or a
> very convincing argument is made.

So, current approach of dropping locks during I/O sounds like still
the best solution.

>
>
> >
> >
> >
> >
> > >
> > > >
> > > > I'd also like to get Suren's input, however.
> > >
> > > Yes. of course.
> > >
> > > >
> > > > Thanks, Lorenzo
> > >
> > > Best Regards
> > > Barry
>
> Cheers, Lorenzo