[PATCH] mm/madvise: swap in CoW'd MAP_PRIVATE-file mappings on MADV_WILLNEED

[Lorenzo Stoakes (ARM)]

Currently MADV_WILLNEED treats file-backed and pure anonymous mappings
entirely separately - using POSIX_FADV_WILLNEED (equivalent of a readahead)
for the former and a tree walk and swap in to swap cache for the latter.

MAP_PRIVATE-file backed mappings straddle the two and currently get treated
as if they were purely file-backed, meaning any swapped out private pages
remain swapped out.

Resolve the issue by explicitly checking for CoW'd MAP_PRIVATE-file backed
mappings and performing both walks in this case.

Since the logic checks for vma->anon_vma this means un-CoW'd
MAP_PRIVATE-file backed mappings retain only the single file walk.

Reported-by: Mike Kaplinskiy <mike@xxxxxxxxx>
Closes: https://lore.kernel.org/all/CABeknB_S2XJSHFgnHdgnN0rjzHhH4oQJs_APq9fvxHztQ_pgiA@xxxxxxxxxxxxxx/
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/madvise.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 73c2901b9adb..963337f93a7a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -297,11 +297,12 @@ static long madvise_willneed(struct madvise_behavior *madv_behavior)
 	loff_t offset;

 #ifdef CONFIG_SWAP
-	if (!file) {
+	if (vma_is_cow_mapping(vma) && vma->anon_vma) {
 		walk_page_range_vma(vma, start, end, &swapin_walk_ops, vma);
 		lru_add_drain(); /* Push any new pages onto the LRU now */
-		return 0;
 	}
+	if (!file)
+		return 0;

 	if (shmem_mapping(file->f_mapping)) {
 		shmem_swapin_range(vma, start, end, file->f_mapping);
--
2.55.0