mm: distinguish large folio swap allocation failures

folio_alloc_swap() reports most failures with generic negative error
codes.  Reclaim callers consequently cannot tell whether splitting a large
folio could make progress, or whether no swap space is available for even
a single page.

Classify failures using both the global free swap count and the remaining
capacity in the folio's memcg swap hierarchy.  Return -ENOSPC when global
swap space is exhausted, -ENOMEM when splitting cannot overcome the
failure, and -E2BIG for a large folio when allocating or charging a
smaller folio might still succeed.

Use this classification for all folio_alloc_swap() failure paths,
including capability rejection, swap slot allocation failure, and memcg
swap charge failure.  Callers are updated separately to split large folios
only on -E2BIG.

Link: https://lore.kernel.org/20260830042920.2280454-3-xueyuan.chen21@gmail.com
Signed-off-by: Xueyuan Chen <xueyuan.chen21@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Kairui Song <ryncsn@gmail.com>
Suggested-by: Barry Song <baohua@kernel.org>
Suggested-by: Youngjun Park <youngjun.park@lge.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 5658a1634b85..7a43409879ca 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -508,6 +508,7 @@ static inline void mem_cgroup_uncharge_swap(unsigned short id, unsigned int nr_p
 	__mem_cgroup_uncharge_swap(id, nr_pages);
 }
 
+long mem_cgroup_get_folio_swap_margin(struct folio *folio);
 extern long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg);
 extern bool mem_cgroup_swap_full(struct folio *folio);
 #else
@@ -521,6 +522,11 @@ static inline void mem_cgroup_uncharge_swap(unsigned short id,
 {
 }
 
+static inline long mem_cgroup_get_folio_swap_margin(struct folio *folio)
+{
+	return PAGE_COUNTER_MAX;
+}
+
 static inline long mem_cgroup_get_nr_swap_pages(struct mem_cgroup *memcg)
 {
 	return get_nr_swap_pages();