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

[Yang Shi]

On Sun, May 17, 2026 at 1:45 AM Barry Song <baohua at kernel.org> wrote:
>
> On Sat, May 2, 2026 at 1:58 AM Matthew Wilcox <willy at infradead.org> wrote:
> >
> > On Sat, May 02, 2026 at 01:44:34AM +0800, Barry Song wrote:
> > > On Fri, May 1, 2026 at 10:57 PM Matthew Wilcox <willy at infradead.org> wrote:
> > > >
> > > > On Fri, May 01, 2026 at 06:49:58AM +0800, Barry Song wrote:
> > > > > 1. There is no deterministic latency for I/O completion. It depends on
> > > > > both the hardware and the software stack (bio/request queues and the
> > > > > block scheduler). Sometimes the latency is short; at other times it can
> > > > > be quite long. In such cases, a high-priority thread performing operations
> > > > > such as mprotect, unmap, prctl_set_vma, or madvise may be forced to wait
> > > > > for an unpredictable amount of time.
> > > >
> > > > But does that actually happen?  I find it hard to believe that thread A
> > > > unmaps a VMA while thread B is in the middle of taking a page fault in
> > > > that same VMA.  mprotect() and madvise() are more likely to happen, but
> > > > it still seems really unlikely to me.
> > >
> > > It doesn’t have to involve unmapping or applying mprotect to
> > > the entire VMA—just a portion of it is sufficient.
> >
> > Yes, but that still fails to answer "does this actually happen".  How much
> > performance is all this complexity in the page fault handler buying us?
> > If you don't answer this question, I'm just going to go in and rip it
> > all out.
> >
>
> Hi Matthew (and Lorenzo, Jan, and anyone else who may be
> waiting for answers),
>
> As promised during LSF/MM/BPF, we conducted thorough
> testing on Android phones to determine whether performing
> I/O in `filemap_fault()` can block `vma_start_write()`.
> I wanted to give a quick update on this question.
>
> Nanzhe at Xiaomi created tracing scripts and ran various
> applications on Android devices with I/O performed under
> the VMA lock in `filemap_fault()`. We found that:
>
> 1. There are very few cases where unmap() is blocked by
>    page faults. I assume this is due to buggy user code
>    or poor synchronization between reads and unmap().
> So I assume it is not a problem.
>
> 2. We observed many cases where `vma_start_write()`
>    is blocked by page-fault I/O in some applications.
>    The blocking occurs in the `dup_mmap()` path during
>    fork().
>
> With Suren's commit fb49c455323ff ("fork: lock VMAs of
> the parent process when forking"), we now always hold
> `vma_write_lock()` for each VMA. Note that the
> `mmap_lock` write lock is also held, which could lead to
> chained waiting if page-fault I/O is performed without
> releasing the VMA lock.
>
> My gut feeling is that Suren's commit may be overshooting,
> so my rough idea is that we might want to do something like
> the following (we haven't tested it yet and it might be
> wrong):
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2311ae7c2ff4..5ddaf297f31a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1762,7 +1762,13 @@ __latent_entropy int dup_mmap(struct mm_struct
> *mm, struct mm_struct *oldmm)
>         for_each_vma(vmi, mpnt) {
>                 struct file *file;
>
> -               retval = vma_start_write_killable(mpnt);
> +               /*
> +                * For anonymous or writable private VMAs, prevent
> +                * concurrent CoW faults.
> +                */
> +               if (!mpnt->vm_file || (!(mpnt->vm_flags & VM_SHARED) &&
> +                                       (mpnt->vm_flags & VM_WRITE)))
> +                       retval = vma_start_write_killable(mpnt);
>                 if (retval < 0)
>                         goto loop_out;
>                 if (mpnt->vm_flags & VM_DONTCOPY) {

Maybe a little bit off topic. This is an interesting idea. It seems
possible we don't have to take vma write lock unconditionally. IIUC
the write lock is mainly used to serialize against page fault and
madvise, right? I got a crazy idea off the top of my head. We may be
able to just take vma write lock iff vma->anon_vma is not NULL.

First of all, write mmap_lock is held, so the vma can't go or be
changed under us.

Secondly, if vma->anon_vma is NULL, it basically means either no page
fault happened or no cow happened, so there is no page table to copy,
this is also what copy_page_range() does currently. So we can shrink
the critical section to:

if (vma->anon_vma) {
    vma_start_write_killable(src_vma);
    anon_vma_fork(dst_vma, src_vma);
    copy_page_range(dst_vma, src_vma);
}

But page fault can happen before write mmap_lock is taken, when we
check vma->anon_vma, it is possible it has not been set up yet. But it
seems to be equivalent to page fault after fork and won't break the
semantic.

Anyway, just a crazy idea, I may miss some corner cases.

Thanks,
Yang

}

>
> Based on the above, we may want to re-check whether fork()
> can be blocked by page faults. At the same time, if Suren,
> you, or anyone else has any comments, please feel free to
> share them.
>
> Best Regards
> Barry
>