mm/filemap: map entire large folio faultaround

Currently, kernel only maps part of large folio that fits into
start_pgoff/end_pgoff range.

Map entire folio where possible.  It will match finish_fault() behaviour
that user hits on cold page cache.

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/20250923110711.690639-6-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Cc: 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>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kiryl Shutsemau

mm/filemap.c

@@ -3670,6 +3670,21 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 	struct page *page = folio_page(folio, start);
 	unsigned int count = 0;
 	pte_t *old_ptep = vmf->pte;
+	unsigned long addr0;
+
+	/*
+	 * Map the large folio fully where possible.
+	 *
+	 * The folio must not cross VMA or page table boundary.
+	 */
+	addr0 = addr - start * PAGE_SIZE;
+	if (folio_within_vma(folio, vmf->vma) &&
+	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK)) {
+		vmf->pte -= start;
+		page -= start;
+		addr = addr0;
+		nr_pages = folio_nr_pages(folio);
+	}
 
 	do {
 		if (PageHWPoison(page + count))