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+0800 Baolin Wang 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.