mm/rmap: mlock large folios in try_to_unmap_one()

Currently, try_to_unmap_once() only tries to mlock small folios.

Use logic similar to folio_referenced_one() to mlock large folios: only do
this for fully mapped folios and under page table lock that protects all
page table entries.

[akpm@linux-foundation.org: s/CROSSSED/CROSSED/]
Link: https://lkml.kernel.org/r/20250923110711.690639-4-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Kiryl Shutsemau

mm/rmap.c

@@ -1870,6 +1870,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 	unsigned long nr_pages = 1, end_addr;
 	unsigned long pfn;
 	unsigned long hsz = 0;
+	int ptes = 0;
 
 	/*
 	 * When racing against e.g. zap_pte_range() on another cpu,
@@ -1910,10 +1911,34 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 		 */
 		if (!(flags & TTU_IGNORE_MLOCK) &&
 		    (vma->vm_flags & VM_LOCKED)) {
+			ptes++;
+
+			/*
+			 * Set 'ret' to indicate the page cannot be unmapped.
+			 *
+			 * Do not jump to walk_abort immediately as additional
+			 * iteration might be required to detect fully mapped
+			 * folio an mlock it.
+			 */
+			ret = false;
+
+			/* Only mlock fully mapped pages */
+			if (pvmw.pte && ptes != pvmw.nr_pages)
+				continue;
+
+			/*
+			 * All PTEs must be protected by page table lock in
+			 * order to mlock the page.
+			 *
+			 * If page table boundary has been cross, current ptl
+			 * only protect part of ptes.
+			 */
+			if (pvmw.flags & PVMW_PGTABLE_CROSSED)
+				goto walk_done;
+
 			/* Restore the mlock which got missed */
-			if (!folio_test_large(folio))
-				mlock_vma_folio(folio, vma);
-			goto walk_abort;
+			mlock_vma_folio(folio, vma);
+			goto walk_done;
 		}
 
 		if (!pvmw.pte) {