[PATCH 5.15.y 6.1.y 6.6.y 1/1] mm/vmscan: flush deferred TLB before freeing large folios

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

In reclaim, shrink_folio_list() unmaps PTEs with a deferred, batched TLB
flush. The batch is only flushed by try_to_unmap_flush() near the end of
the function, just before the order-0 folios collected in @free_folios are
handed back to the allocator.

Large folios don't go through @free_folios -- they're freed inline at the
free_it label via destroy_large_folio(), which runs before that flush. So
a large folio's pages can be returned to the buddy allocator and reused
while another CPU still holds a stale TLB entry for them, and that CPU then
reads or executes through the stale translation into the reused page. For
file-backed large folios (e.g. executable text) this shows up as random
SIGSEGV/SIGILL in user space, with fault addresses that don't match the
code being run.

Flush the deferred batch before freeing a large folio inline, the same way
the order-0 path already waits for the flush.

Upstream this is fixed as a side effect of commit bc2ff4cbc329 ("mm: free
folios in a batch in shrink_folio_list()"), which is a larger change; this
is the minimal fix for -stable.

Reported-by: Yingfu Zhou <yingfu.zhou@shopee.com>
Fixes: bd4c82c22c36 ("mm, THP, swap: delay splitting THP after swapped out")
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
destroy_compound_page was recently renamed to destroy_large_folio.
So it would be conflict when this patch was applied to 5.15/6.1
---
 mm/vmscan.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index aba757e5c597..8eb498351d9b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2123,10 +2123,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 		 * Is there need to periodically free_folio_list? It would
 		 * appear not as the counts should be low
 		 */
-		if (unlikely(folio_test_large(folio)))
+		if (unlikely(folio_test_large(folio))) {
+			try_to_unmap_flush();
 			destroy_large_folio(folio);
-		else
+		} else {
 			list_add(&folio->lru, &free_folios);
+		}
 		continue;
 
 activate_locked_split:
-- 
2.43.0