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

David Hildenbrand
2025-09-18T11:30:41Z

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