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

[Pedro Falcato]

On Thu, Nov 27, 2025 at 07:39:11PM +0800, Barry Song wrote:
> On Thu, Nov 27, 2025 at 6:52 PM Pedro Falcato <pfalcato at suse.de> wrote:
> >
> > On Thu, Nov 27, 2025 at 09:14:37AM +0800, Barry Song wrote:
> > > From: Oven Liyang <liyangouwen1 at 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 at oppo.com>
> > > Signed-off-by: Barry Song <v-songbaohua at 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.
>

Hmm, right, good point. I think this is the safest option then.

FWIW:
Acked-by: Pedro Falcato <pfalcato at suse.de>

-- 
Pedro