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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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@recall.ai>
Closes: https://lore.kernel.org/all/CABeknB_S2XJSHFgnHdgnN0rjzHhH4oQJs_APq9fvxHztQ_pgiA@mail.gmail.com/
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/madvise.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index 73c2901b9adb..d0510dd49dde 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -297,10 +297,12 @@ static long madvise_willneed(struct madvise_behavior *madv_behavior)
 	loff_t offset;
 
 #ifdef CONFIG_SWAP
-	if (!file) {
+	if (!file || (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)) {

---
base-commit: e3b5239afe1b8f0194db7436b17c33e94c1988c4
change-id: 20260903-madv-will-need-map-private-125e3e2f70c7

Best regards,
-- 
Lorenzo Stoakes (ARM) <ljs@kernel.org>