[PATCH v4 4/4] mm: swap: filter swap allocation by memcg tier mask

[Youngjun Park <youngjun.park@lge.com>]

Apply memcg tier effective mask during swap slot allocation to
enforce per-cgroup swap tier restrictions.

In the fast path, check the percpu cached swap_info's tier_mask
against the folio's effective mask. If it does not match, fall
through to the slow path. In the slow path, skip swap devices
whose tier_mask is not covered by the folio's effective mask.

This works correctly when there is only one non-rotational
device in the system and no devices share the same priority.
However, there are known limitations:

 - When multiple non-rotational devices exist, percpu swap
   caches from different memcg contexts may reference
   mismatched tiers, causing unnecessary fast path misses.

 - When multiple non-rotational devices are assigned to
   different tiers and same-priority devices exist among
   them, cluster-based rotation may not work correctly.

These edge cases do not affect the primary use case of
directing swap traffic per cgroup. Further optimization is
planned for future work.

Signed-off-by: Youngjun Park <youngjun.park@lge.com>
---
 mm/swapfile.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2f956b6a5edc..aff5e8407691 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1321,15 +1321,22 @@ static bool swap_alloc_fast(struct folio *folio)
 	struct swap_cluster_info *ci;
 	struct swap_info_struct *si;
 	unsigned int offset;
+	int mask = folio_tier_effective_mask(folio);
 
 	/*
 	 * Once allocated, swap_info_struct will never be completely freed,
 	 * so checking it's liveness by get_swap_device_info is enough.
 	 */
 	si = this_cpu_read(percpu_swap_cluster.si[order]);
+	if (!si || !swap_tiers_mask_test(si->tier_mask, mask) ||
+		!get_swap_device_info(si))
+		return false;
+
 	offset = this_cpu_read(percpu_swap_cluster.offset[order]);
-	if (!si || !offset || !get_swap_device_info(si))
+	if (!offset) {
+		put_swap_device(si);
 		return false;
+	}
 
 	ci = swap_cluster_lock(si, offset);
 	if (cluster_is_usable(ci, order)) {
@@ -1348,10 +1355,14 @@ static bool swap_alloc_fast(struct folio *folio)
 static void swap_alloc_slow(struct folio *folio)
 {
 	struct swap_info_struct *si, *next;
+	int mask = folio_tier_effective_mask(folio);
 
 	spin_lock(&swap_avail_lock);
 start_over:
 	plist_for_each_entry_safe(si, next, &swap_avail_head, avail_list) {
+		if (!swap_tiers_mask_test(si->tier_mask, mask))
+			continue;
+
 		/* Rotate the device and switch to a new cluster */
 		plist_requeue(&si->avail_list, &swap_avail_head);
 		spin_unlock(&swap_avail_lock);
-- 
2.34.1