Re: [RFC PATCH 1/2] mm/filemap: Retry fault by VMA lock if the lock was released for I/O

[Barry Song <21cnbao@gmail.com>]

On Thu, Nov 27, 2025 at 6:52 PM Pedro Falcato <pfalcato@suse.de> wrote:
>
> On Thu, Nov 27, 2025 at 09:14:37AM +0800, Barry Song wrote:
> > From: Oven Liyang <liyangouwen1@oppo.com>
> >
> > If the current page fault is using the per-VMA lock, and we only released
> > the lock to wait for I/O completion (e.g., using folio_lock()), then when
> > the fault is retried after the I/O completes, it should still qualify for
> > the per-VMA-lock path.
> >
> <snip>
> > Signed-off-by: Oven Liyang <liyangouwen1@oppo.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  arch/arm/mm/fault.c       | 5 +++++
> >  arch/arm64/mm/fault.c     | 5 +++++
> >  arch/loongarch/mm/fault.c | 4 ++++
> >  arch/powerpc/mm/fault.c   | 5 ++++-
> >  arch/riscv/mm/fault.c     | 4 ++++
> >  arch/s390/mm/fault.c      | 4 ++++
> >  arch/x86/mm/fault.c       | 4 ++++
>
> If only we could unify all these paths :(

Right, it’s a pain, but we do have bots for that?
And it’s basically just copy-and-paste across different architectures.

>
> >  include/linux/mm_types.h  | 9 +++++----
> >  mm/filemap.c              | 5 ++++-
> >  9 files changed, 39 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index b71625378ce3..12b2d65ef1b9 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1670,10 +1670,11 @@ enum vm_fault_reason {
> >       VM_FAULT_NOPAGE         = (__force vm_fault_t)0x000100,
> >       VM_FAULT_LOCKED         = (__force vm_fault_t)0x000200,
> >       VM_FAULT_RETRY          = (__force vm_fault_t)0x000400,
> > -     VM_FAULT_FALLBACK       = (__force vm_fault_t)0x000800,
> > -     VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
> > -     VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
> > -     VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
> > +     VM_FAULT_RETRY_VMA      = (__force vm_fault_t)0x000800,
>
> So, what I am wondering here is why we need one more fault flag versus
> just blindly doing this on a plain-old RETRY. Is there any particular
> reason why? I can't think of one.

Because in some cases we retry simply due to needing to take mmap_lock.
For example:

/**
 * __vmf_anon_prepare - Prepare to handle an anonymous fault.
 * @vmf: The vm_fault descriptor passed from the fault handler.
 *
 * When preparing to insert an anonymous page into a VMA from a
 * fault handler, call this function rather than anon_vma_prepare().
 * If this vma does not already have an associated anon_vma and we are
 * only protected by the per-VMA lock, the caller must retry with the
 * mmap_lock held.  __anon_vma_prepare() will look at adjacent VMAs to
 * determine if this VMA can share its anon_vma, and that's not safe to
 * do with only the per-VMA lock held for this VMA.
 *
 * Return: 0 if fault handling can proceed.  Any other value should be
 * returned to the caller.
 */
vm_fault_t __vmf_anon_prepare(struct vm_fault *vmf)
{
...
}

Thus, we have to check each branch one by one, but I/O wait is the most
frequent path, so we handle it first.

>
> I would also like to see performance numbers.

Yes. From what I understand, this patchset should improve performance in a
fairly straightforward way.
But yes, I can certainly include some data in v2.

>
> The rest of the patch looks OK to me.

Thanks
Barry