mm/fault: try to map the entire file folio in finish_fault()

finish_fault() 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.

Link: https://lkml.kernel.org/r/20250923110711.690639-5-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 17cebb97beaeab334abed179b3b005caf5c4554d..74b45e258323e5c40c4f5d6955d60df87c9a0eb6 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5516,13 +5516,8 @@ fallback:
 
        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);