Re: [PATCH 1/2] mm/fault: Try to map the entire file folio in finish_fault()

Lorenzo Stoakes
2025-09-18T13:19:28Z

On Thu, Sep 18, 2025 at 01:30:32PM +0200, David Hildenbrand wrote:
> On 18.09.25 13:21, kirill@shutemov.name wrote:
> > From: Kiryl Shutsemau <kas@kernel.org>
> >
> > The finish_fault() function uses per-page fault for file folios. This
> > only occurs for file folios smaller than PMD_SIZE.
> >
> > The comment suggests that this approach prevents RSS inflation.
> > However, it only prevents RSS accounting. The folio is still mapped to
> > the process, and the fact that it is mapped by a single PTE does not
> > affect memory pressure. Additionally, the kernel's ability to map
> > large folios as PMD if they are large enough does not support this
> > argument.
> >
> > When possible, map large folios in one shot. This reduces the number of
> > minor page faults and allows for TLB coalescing.
> >
> > Mapping large folios at once will allow the rmap code to mlock it on
> > add, as it will recognize that it is fully mapped and mlocking is safe.
> >
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > ---
> > mm/memory.c | 9 ++-------
> > 1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0ba4f6b71847..812a7d9f6531 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5386,13 +5386,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> > nr_pages = folio_nr_pages(folio);
> > - /*
> > - * Using per-page fault to maintain the uffd semantics, and same
> > - * approach also applies to non shmem/tmpfs faults to avoid
> > - * inflating the RSS of the process.
> > - */
> > - if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> > - unlikely(needs_fallback)) {
> > + /* Using per-page fault to maintain the uffd semantics */
> > + if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) {
> > nr_pages = 1;
> > } else if (nr_pages > 1) {
> > pgoff_t idx = folio_page_idx(folio, page);
>
> I could have sworn that we recently discussed that.
>
> Ah yes, there it is
>
> https://lkml.kernel.org/r/a1c9ba0f-544d-4204-ad3b-60fe1be2ab32@linux.alibaba.com
>
> CCing Baolin as he wanted to look into this.
>
> --
> Cheers
>
> David / dhildenb
>

Yeah Baolin already did work here [0] so let's get his input first I think! :)

[0]:https://lore.kernel.org/linux-mm/440940e78aeb7430c5cc8b6d2088ae98265b9809.1751599072.git.baolin.wang@linux.alibaba.com/