[PATCH v2] mm: memcg-v1: fix memsw and TCP failcnt accounting

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

Commit 0e2759afcaf9 ("page_counter: track failcnt only for legacy
cgroups") made failcnt accounting conditional on track_failcnt. It
enabled the flag for memcg->memory, but not for memcg->memsw or
memcg->tcpmem.

Consequently, memory.memsw.failcnt remains zero when the memory+swap
limit is hit. memory.kmem.tcp.limit_in_bytes still sets
memcg->tcpmem.max, but TCP charge failures are not reflected in
memory.kmem.tcp.failcnt.

Enable failcnt accounting for both v1 counters.

To reproduce memory.memsw.failcnt:

    CG=/sys/fs/cgroup/memory/memsw-test
    LIMIT=33554432
    mkdir "$CG"
    echo "$LIMIT" > "$CG/memory.limit_in_bytes"
    echo "$LIMIT" > "$CG/memory.memsw.limit_in_bytes"

Start a child process in the cgroup and make it allocate and touch 96 MiB
of memory, causing a memcg OOM.

    cat "$CG/memory.memsw.failcnt"

Without the patch, memory.memsw.failcnt is 0. With the patch,
memory.memsw.failcnt is greater than 0.

To reproduce memory.kmem.tcp.failcnt:

    CG=/sys/fs/cgroup/memory/tcpmem-test
    LIMIT=65536
    mkdir "$CG"
    echo "$LIMIT" > "$CG/memory.kmem.tcp.limit_in_bytes"

Start a child process in the cgroup, create a TCP socket, and reserve
1 MiB of socket memory with SO_RESERVE_MEM. The reservation fails with
ENOMEM.

    cat "$CG/memory.kmem.tcp.failcnt"

Without the patch, memory.kmem.tcp.failcnt is 0. With the patch,
memory.kmem.tcp.failcnt is greater than 0.

Closes: https://sashiko.dev/#/patchset/20260810074247.52747-1-guopeng.zhang@linux.dev?part=1
Fixes: 0e2759afcaf9 ("page_counter: track failcnt only for legacy cgroups")
Cc: stable@vger.kernel.org
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
Changes in v2:
- Enable failcnt tracking for tcpmem as suggested by Johannes Weiner.
- Add the memory.kmem.tcp.failcnt reproducer and test result.
- Update the subject and changelog to cover both counters.
- Add Cc: stable@vger.kernel.org and the received Acked-by tags.

Link to v1: https://lore.kernel.org/r/20260810074247.52747-1-guopeng.zhang@linux.dev

 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 70806373d5a9..17da1f43b7d3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4235,9 +4235,11 @@ mem_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 #ifdef CONFIG_MEMCG_V1
 		WRITE_ONCE(memcg->swappiness, mem_cgroup_swappiness(parent));
 		memcg->memory.track_failcnt = !memcg_on_dfl;
+		memcg->memsw.track_failcnt = !memcg_on_dfl;
 		WRITE_ONCE(memcg->oom_kill_disable, READ_ONCE(parent->oom_kill_disable));
 		page_counter_init(&memcg->kmem, &parent->kmem, false);
 		page_counter_init(&memcg->tcpmem, &parent->tcpmem, false);
+		memcg->tcpmem.track_failcnt = !memcg_on_dfl;
 #endif
 	} else {
 		init_memcg_stats();
-- 
2.43.0