[PATCH 3/4] mm, swap: only allow swapped-out slots into the swap cache

[Youngjun Park]

__swap_cache_add_check() turns away folio entries and slots with no count
and lets everything else in.  That is safe only when the caller owns the
slot.  Cluster readahead owns nothing, it walks a raw page_cluster sized
window of offsets around the faulting entry.

A hibernation slot is not a folio, and the count test does not stop it
either, because the type the previous patch added has the count bits set.
So readahead allocates a folio and reads the offset off the device into
it, for a slot that nothing will ever swap in.  A bad slot listed in the
swap header gets in the same way, its count bits are set too, and the folio
entry that replaces it drops the bad marker.  The first patch keeps that
folio from doing harm, but the folio and the read still happen.

Require a shadow entry instead.  A slot dropped from the swap cache always
gets one, empty if there is no workingset value.  The check runs before the
folio allocation in __swap_cache_alloc(), so readahead now skips the offset
without allocating or reading.

Signed-off-by: Youngjun Park <youngjun.park@xxxxxxx>
---
 mm/swap_state.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 5be825911e64..9f2cc5918713 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -180,9 +180,14 @@ static int __swap_cache_add_check(struct swap_cluster_info *ci,
 	old_tb = __swap_table_get(ci, ci_off);
 	if (swp_tb_is_folio(old_tb))
 		return -EEXIST;
-	if (!__swp_tb_get_count(old_tb))
+	/*
+	 * Only a swapped-out slot may be brought into the swap cache.
+	 * Cluster readahead walks raw offset ranges, so it can land on
+	 * slots that are free, bad, or owned by hibernation.
+	 */
+	if (!swp_tb_is_shadow(old_tb) || !__swp_tb_get_count(old_tb))
 		return -ENOENT;
-	if (shadowp && swp_tb_is_shadow(old_tb))
+	if (shadowp)
 		*shadowp = swp_tb_to_shadow(old_tb);
 	if (memcg_id)
 		*memcg_id = __swap_cgroup_get(ci, ci_off);
-- 
2.48.1