1 files changed, 43 insertions, 6 deletions

diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index e485b828d173..b8edb9f981d7 100644
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -146,19 +146,56 @@ static int memfd_luo_preserve_folios(struct file *file,
 	for (i = 0; i < nr_folios; i++) {
 		struct memfd_luo_folio_ser *pfolio = &folios_ser[i];
 		struct folio *folio = folios[i];
-		unsigned int flags = 0;
 
 		err = kho_preserve_folio(folio);
 		if (err)
 			goto err_unpreserve;
 
-		if (folio_test_dirty(folio))
-			flags |= MEMFD_LUO_FOLIO_DIRTY;
-		if (folio_test_uptodate(folio))
-			flags |= MEMFD_LUO_FOLIO_UPTODATE;
+		folio_lock(folio);
+
+		/*
+		 * A dirty folio is one which has been written to. A clean folio
+		 * is its opposite. Since a clean folio does not carry user
+		 * data, it can be freed by page reclaim under memory pressure.
+		 *
+		 * Saving the dirty flag at prepare() time doesn't work since it
+		 * can change later. Saving it at freeze() also won't work
+		 * because the dirty bit is normally synced at unmap and there
+		 * might still be a mapping of the file at freeze().
+		 *
+		 * To see why this is a problem, say a folio is clean at
+		 * preserve, but gets dirtied later. The pfolio flags will mark
+		 * it as clean. After retrieve, the next kernel might try to
+		 * reclaim this folio under memory pressure, losing user data.
+		 *
+		 * Unconditionally mark it dirty to avoid this problem. This
+		 * comes at the cost of making clean folios un-reclaimable after
+		 * live update.
+		 */
+		folio_mark_dirty(folio);
+
+		/*
+		 * If the folio is not uptodate, it was fallocated but never
+		 * used. Saving this flag at prepare() doesn't work since it
+		 * might change later when someone uses the folio.
+		 *
+		 * Since we have taken the performance penalty of allocating,
+		 * zeroing, and pinning all the folios in the holes, take a bit
+		 * more and zero all non-uptodate folios too.
+		 *
+		 * NOTE: For someone looking to improve preserve performance,
+		 * this is a good place to look.
+		 */
+		if (!folio_test_uptodate(folio)) {
+			folio_zero_range(folio, 0, folio_size(folio));
+			flush_dcache_folio(folio);
+			folio_mark_uptodate(folio);
+		}
+
+		folio_unlock(folio);
 
 		pfolio->pfn = folio_pfn(folio);
-		pfolio->flags = flags;
+		pfolio->flags = MEMFD_LUO_FOLIO_DIRTY | MEMFD_LUO_FOLIO_UPTODATE;
 		pfolio->index = folio->index;
 	}