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>

mm/memory.c

@@ -5516,13 +5516,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 
 	nr_pages = folio_nr_pages(folio);
 
-	/*
+	/* Using per-page fault to maintain the uffd semantics */
-	 * Using per-page fault to maintain the uffd semantics, and same
+	if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) {
-	 * 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)) {
 		nr_pages = 1;
 	} else if (nr_pages > 1) {
 		pgoff_t idx = folio_page_idx(folio, page);