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.com
Signed-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>

diff --git a/mm/ksm.c b/mm/ksm.c
index 742b005f3f77b4a0bc5e892a0475c99b7f3c7870..0f5b2bba4ef0775fa7d61742589482e7312fa9f0 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2564,36 +2564,46 @@ next_mm:
                        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();
                }