Re: [RFC PATCH v5] mm: retry page faults once under the per-VMA lock

[Hongru Zhang]

On Sat, Aug 15, 2026 at 2:51 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 14 Aug 2026 16:53:00 +0800 Hongru Zhang <zhanghongru06@xxxxxxxxx> wrote:
>
> > From: Hongru Zhang <zhanghongru@xxxxxxxxxx>
> >
> > The per-VMA lock fault path falls back to mmap_lock on VM_FAULT_RETRY.
> > When mmap_lock is write-contended, the fallback can reduce page-fault
> > throughput.
> >
> > Add a single retry under the per-VMA lock in the arch fault handler
> > instead of adding a new VM_FAULT_* flag.
> >
> > ...
> >
> > Swap Throughput (higher is better):
> > +--------------+-------------+---------------------------+
> > | mmap writers |   Vanilla   |          Patched          |
> > +--------------+-------------+---------------------------+
> > |      0       | 17303.09 /s | 17899.48 /s       (+3.4%) |
> > +--------------+-------------+---------------------------+
> > |      4       | 12596.23 /s | 16095.20 /s      (+27.8%) |
> > +--------------+-------------+---------------------------+
> > |      8       |     0.58 /s | 15420.57 /s (+2658619.0%) |
> > +--------------+-------------+---------------------------+
>
> Well.
>
> > With increasing mmap_lock write pressure, Vanilla degrades sharply and
> > drops to near zero at eight writers. Patched kernel holds up much better.
>
> Thanks for working on this.
>
> Are you able to help us understand how much effect this change will
> have upon any real-world workloads?

Thanks for raising this.

I'll run some real-world workload tests and follow up with the results.