+CC Shakeel

On Wed, Aug 23, 2023 at 03:54:30PM -0700, Nhat Pham wrote:

From: Johannes Weiner hannes@cmpxchg.org

In the eviction recency check, we attempt to retrieve the memcg to which the folio belonged when it was evicted, by the memcg id stored in the shadow entry. However, there is a chance that the retrieved memcg is not the original memcg that has been killed, but a new one which happens to have the same id.

This is a somewhat unfortunate, but acceptable and rare inaccuracy in the heuristics. However, if we retrieve this new memcg between its allocation and when it is properly attached to the memcg hierarchy, we could run into the following NULL pointer exception during the memcg hierarchy traversal done in mem_cgroup_get_nr_swap_pages():

[ 155757.793456] BUG: kernel NULL pointer dereference, address: 00000000000000c0 [ 155757.807568] #PF: supervisor read access in kernel mode [ 155757.818024] #PF: error_code(0x0000) - not-present page [ 155757.828482] PGD 401f77067 P4D 401f77067 PUD 401f76067 PMD 0 [ 155757.839985] Oops: 0000 [#1] SMP [ 155757.887870] RIP: 0010:mem_cgroup_get_nr_swap_pages+0x3d/0xb0 [ 155757.899377] Code: 29 19 4a 02 48 39 f9 74 63 48 8b 97 c0 00 00 00 48 8b b7 58 02 00 00 48 2b b7 c0 01 00 00 48 39 f0 48 0f 4d c6 48 39 d1 74 42 <48> 8b b2 c0 00 00 00 48 8b ba 58 02 00 00 48 2b ba c0 01 00 00 48 [ 155757.937125] RSP: 0018:ffffc9002ecdfbc8 EFLAGS: 00010286 [ 155757.947755] RAX: 00000000003a3b1c RBX: 000007ffffffffff RCX: ffff888280183000 [ 155757.962202] RDX: 0000000000000000 RSI: 0007ffffffffffff RDI: ffff888bbc2d1000 [ 155757.976648] RBP: 0000000000000001 R08: 000000000000000b R09: ffff888ad9cedba0 [ 155757.991094] R10: ffffea0039c07900 R11: 0000000000000010 R12: ffff888b23a7b000 [ 155758.005540] R13: 0000000000000000 R14: ffff888bbc2d1000 R15: 000007ffffc71354 [ 155758.019991] FS: 00007f6234c68640(0000) GS:ffff88903f9c0000(0000) knlGS:0000000000000000 [ 155758.036356] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 155758.048023] CR2: 00000000000000c0 CR3: 0000000a83eb8004 CR4: 00000000007706e0 [ 155758.062473] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 155758.076924] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 155758.091376] PKRU: 55555554 [ 155758.096957] Call Trace: [ 155758.102016] <TASK> [ 155758.106502] ? __die+0x78/0xc0 [ 155758.112793] ? page_fault_oops+0x286/0x380 [ 155758.121175] ? exc_page_fault+0x5d/0x110 [ 155758.129209] ? asm_exc_page_fault+0x22/0x30 [ 155758.137763] ? mem_cgroup_get_nr_swap_pages+0x3d/0xb0 [ 155758.148060] workingset_test_recent+0xda/0x1b0 [ 155758.157133] workingset_refault+0xca/0x1e0 [ 155758.165508] filemap_add_folio+0x4d/0x70 [ 155758.173538] page_cache_ra_unbounded+0xed/0x190 [ 155758.182919] page_cache_sync_ra+0xd6/0x1e0 [ 155758.191738] filemap_read+0x68d/0xdf0 [ 155758.199495] ? mlx5e_napi_poll+0x123/0x940 [ 155758.207981] ? __napi_schedule+0x55/0x90 [ 155758.216095] __x64_sys_pread64+0x1d6/0x2c0 [ 155758.224601] do_syscall_64+0x3d/0x80 [ 155758.232058] entry_SYSCALL_64_after_hwframe+0x46/0xb0 [ 155758.242473] RIP: 0033:0x7f62c29153b5 [ 155758.249938] Code: e8 48 89 75 f0 89 7d f8 48 89 4d e0 e8 b4 e6 f7 ff 41 89 c0 4c 8b 55 e0 48 8b 55 e8 48 8b 75 f0 8b 7d f8 b8 11 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 33 44 89 c7 48 89 45 f8 e8 e7 e6 f7 ff 48 8b [ 155758.288005] RSP: 002b:00007f6234c5ffd0 EFLAGS: 00000293 ORIG_RAX: 0000000000000011 [ 155758.303474] RAX: ffffffffffffffda RBX: 00007f628c4e70c0 RCX: 00007f62c29153b5 [ 155758.318075] RDX: 000000000003c041 RSI: 00007f61d2986000 RDI: 0000000000000076 [ 155758.332678] RBP: 00007f6234c5fff0 R08: 0000000000000000 R09: 0000000064d5230c [ 155758.347452] R10: 000000000027d450 R11: 0000000000000293 R12: 000000000003c041 [ 155758.362044] R13: 00007f61d2986000 R14: 00007f629e11b060 R15: 000000000027d450 [ 155758.376661] </TASK>

This patch fixes the issue by moving the memcg's id publication from the alloc stage to online stage, ensuring that any memcg acquired via id must be connected to the memcg tree.

Fixes: f78dfc7b77d5 ("workingset: fix confusion around eviction vs refault container") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Co-developed-by: Nhat Pham nphamcs@gmail.com Signed-off-by: Nhat Pham nphamcs@gmail.com Cc: stable@vger.kernel.org

---

mm/memcontrol.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c index cf57fe9318d5..6d67a3a81cd9 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5326,7 +5326,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void) INIT_LIST_HEAD(&memcg->deferred_split_queue.split_queue); memcg->deferred_split_queue.split_queue_len = 0; #endif

• idr_replace(&mem_cgroup_idr, memcg, memcg->id.id); lru_gen_init_memcg(memcg); return memcg;

fail: @@ -5398,14 +5397,27 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css) if (alloc_shrinker_info(memcg)) goto offline_kmem;

• /* Online state pins memcg ID, memcg ID pins CSS */
• refcount_set(&memcg->id.ref, 1);
• css_get(css);
• if (unlikely(mem_cgroup_is_root(memcg))) queue_delayed_work(system_unbound_wq, &stats_flush_dwork, FLUSH_TIME); lru_gen_online_memcg(memcg);

• /* Online state pins memcg ID, memcg ID pins CSS */
• refcount_set(&memcg->id.ref, 1);
• css_get(css);
• /*

• * Ensure mem_cgroup_from_id() works once we're fully online.
  

• *
  

• * We could do this earlier and require callers to filter with
  

• * css_tryget_online(). But right now there are no users that
  

• * need earlier access, and the workingset code relies on the
  

• * cgroup tree linkage (mem_cgroup_get_nr_swap_pages()). So
  

• * publish it here at the end of onlining. This matches the
  

• * regular ID destruction during offlining.
  

• */
  

• idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
• return 0;

offline_kmem: memcg_offline_kmem(memcg); -- 2.34.1