Re: [PATCH v2] mm: fault in complete folios instead of individual pages for tmpfs

[Lorenzo Stoakes]

On Sun, Jul 06, 2025 at 10:02:35AM +0800, Baolin Wang wrote:
>
>
> On 2025/7/5 06:18, Andrew Morton wrote:
> > On Fri,  4 Jul 2025 11:19:26 +0800 Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> > > After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"),
> > > tmpfs can also support large folio allocation (not just PMD-sized large
> > > folios).
> > >
> > > However, when accessing tmpfs via mmap(), although tmpfs supports large folios,
> > > we still establish mappings at the base page granularity, which is unreasonable.
> > >
> > > We can map multiple consecutive pages of a tmpfs folios at once according to
> > > the size of the large folio. On one hand, this can reduce the overhead of page
> > > faults; on the other hand, it can leverage hardware architecture optimizations
> > > to reduce TLB misses, such as contiguous PTEs on the ARM architecture.
> > >
> > > Moreover, tmpfs mount will use the 'huge=' option to control large folio
> > > allocation explicitly. So it can be understood that the process's RSS statistics
> > > might increase, and I think this will not cause any obvious effects for users.
> > >
> > > Performance test:
> > > I created a 1G tmpfs file, populated with 64K large folios, and write-accessed it
> > > sequentially via mmap(). I observed a significant performance improvement:
> >
> > That doesn't sound like a crazy thing to do.
> >
> > > Before the patch:
> > > real	0m0.158s
> > > user	0m0.008s
> > > sys	0m0.150s
> > >
> > > After the patch:
> > > real	0m0.021s
> > > user	0m0.004s
> > > sys	0m0.017s
> >
> > And look at that.
> >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 0f9b32a20e5b..9944380e947d 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -5383,10 +5383,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> > >   	/*
> > >   	 * Using per-page fault to maintain the uffd semantics, and same
> > > -	 * approach also applies to non-anonymous-shmem faults to avoid
> > > +	 * approach also applies to non shmem/tmpfs faults to avoid
> > >   	 * inflating the RSS of the process.
> > >   	 */
> > > -	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> > > +	if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> > >   	    unlikely(needs_fallback)) {
> > >   		nr_pages = 1;
> > >   	} else if (nr_pages > 1) {
> >
> > and that's it?
> >
> > I'm itching to get this into -stable, really.  What LTS user wouldn't
> > want this?
>
> This is an improvement rather than a bugfix, so I don't think it needs to go
> into LTS.
>
> Could it be viewed as correcting an oversight in
> > acd7ccb284b8?
>
> Yes, I should have added this optimization in the series of the commit
> acd7ccb284b8. But obviously, I missed this :(.

Buuut if this was an oversight for that patch that causes an unnecessary
perf degradation, surely this should have fixes tag + cc stable no?

Seems correct to backport to me.