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

kirill
2025-09-18T11:22:05Z

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);
--
2.50.1