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

Barry Song
2026-04-30T22:50:24Z

On Thu, Apr 30, 2026 at 8:37 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Apr 30, 2026 at 12:04:22PM +0800, Barry Song (Xiaomi) wrote:
> > (1) If we need to wait for I/O completion, we still drop the per-VMA lock, as
> > current page fault handling already does. Holding it for too long may introduce
> > various priority inversion issues on mobile devices. After I/O completes, we
> > retry the page fault with the per-VMA lock, rather than falling back to
> > mmap_lock.
>
> You're going to have to do better than that. You know I hate the
> additional complexity you're adding. You need to explain why my idea of
> ripping out all the complexity now that we have per-VMA locks doesn't
> work.

Yep, I know you don’t like the added complexity, but I would rather prioritize
user experience over simplicity. Let me try to explain in more detail.

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. For example, if low-priority tasks
trigger page faults and issue low-priority I/O, a high-priority task
requiring the write lock may end up waiting for an unknown amount of time,
depending on the block layer and filesystem behavior.

As a result, high-priority tasks are exposed to unpredictable I/O latency
introduced by many low-priority tasks that may generate a large number of
page faults.

On Android, latency in certain tasks can significantly affect user experience,
such as interactive threads. Priority inversion is particularly problematic and
should be avoided, especially since we have no clear bound on how long we may
have to wait for I/O from other tasks.

Meanwhile, priority inversion can propagate through a long chain: a writer
waiting on I/O from multiple concurrent page faults may end up blocking other
writers and readers as well. A long-waiting writer can also amplify
mmap_lock contention, which we still rely on in many cases.

2. VMA sizes can be highly uneven: some VMAs may be very large while others are
small. We used to have many reasons to release mmap_lock when we did not have a
per-VMA lock. Since VMA sizes are not uniform, those same considerations may
still apply to the per-VMA lock when a small number of VMAs account for most
of a process’s address space. I recall that Suren also mentioned this[1].

So I would prefer that we hold only the per-VMA lock and avoid retrying the
page fault when we are reasonably sure that I/O has already completed and we
are only waiting for short-lived conditions. Uncertainties in the block layer,
filesystem, and GC behavior, as well as latency-induced priority inversion
chains and potentially amplified mmap_lock contention, can significantly hurt
Android user experience.

[1] https://lore.kernel.org/linux-mm/CAJuCfpFVQJtvbj5fV2fmm4APhNZDL1qPg-YExw7gO1pmngC3Rw@mail.gmail.com/

Thanks
Barry