mm/ksm: convert scan_get_next_rmap_item() from follow_page() to folio_walk

Let's use folio_walk instead, for example avoiding taking temporary folio
references if the folio does obviously not even apply and getting rid of
one more follow_page() user.  We cannot move all handling under the PTL,
so leave the rmap handling (which implies an allocation) out.

Note that zeropages obviously don't apply: old code could just have
specified FOLL_DUMP.  Further, we don't care about losing the secretmem
check in follow_page(): these are never anon pages and
vma_ksm_compatible() would never consider secretmem vmas (VM_SHARED |
VM_MAYSHARE must be set for secretmem, see secretmem_mmap()).

Link: https://lkml.kernel.org/r/20240802155524.517137-7-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/ksm.c

@@ -2564,36 +2564,46 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 			ksm_scan.address = vma->vm_end;
 
 		while (ksm_scan.address < vma->vm_end) {
+			struct page *tmp_page = NULL;
+			struct folio_walk fw;
+			struct folio *folio;
+
 			if (ksm_test_exit(mm))
 				break;
-			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
-			if (IS_ERR_OR_NULL(*page)) {
-				ksm_scan.address += PAGE_SIZE;
-				cond_resched();
-				continue;
+
+			folio = folio_walk_start(&fw, vma, ksm_scan.address, 0);
+			if (folio) {
+				if (!folio_is_zone_device(folio) &&
+				     folio_test_anon(folio)) {
+					folio_get(folio);
+					tmp_page = fw.page;
+				}
+				folio_walk_end(&fw, vma);
 			}
-			if (is_zone_device_page(*page))
-				goto next_page;
-			if (PageAnon(*page)) {
-				flush_anon_page(vma, *page, ksm_scan.address);
-				flush_dcache_page(*page);
+
+			if (tmp_page) {
+				flush_anon_page(vma, tmp_page, ksm_scan.address);
+				flush_dcache_page(tmp_page);
 				rmap_item = get_next_rmap_item(mm_slot,
 					ksm_scan.rmap_list, ksm_scan.address);
 				if (rmap_item) {
 					ksm_scan.rmap_list =
 							&rmap_item->rmap_list;
 
-					if (should_skip_rmap_item(*page, rmap_item))
+					if (should_skip_rmap_item(tmp_page, rmap_item)) {
+						folio_put(folio);
 						goto next_page;
+					}
 
 					ksm_scan.address += PAGE_SIZE;
-				} else
-					put_page(*page);
+					*page = tmp_page;
+				} else {
+					folio_put(folio);
+				}
 				mmap_read_unlock(mm);
 				return rmap_item;
 			}
 next_page:
-			put_page(*page);
 			ksm_scan.address += PAGE_SIZE;
 			cond_resched();
 		}