[PATCH] mm/list_lru: disable memcg awareness under cgroup_disable=memory

[Qinyun Tan <qinyuntan@linux.alibaba.com>]

__list_lru_init() only collapses a memcg-aware list_lru into plain
per-node lists when kmem accounting is disabled
(cgroup.memory=nokmem).  When the memory controller is disabled
entirely (cgroup_disable=memory), mem_cgroup_kmem_disabled() is
false, so the lru stays memcg aware even though no object will ever
be charged to a memcg.

This is more than a semantic inconsistency.
folio_memcg_list_lru_alloc() trusts list_lru_memcg_aware() and
dereferences the folio's memcg, which is always NULL with the
controller disabled.  The only mainline caller,
folio_memcg_alloc_deferred(), papers over this with an explicit
mem_cgroup_disabled() check.  The shmem unused-huge shrinker
conversion ("mm: shmem: make unused huge shrinker memcg aware") adds
a second caller without such a guard, so booting with
cgroup_disable=memory and writing to a huge=always tmpfs oopses:

  BUG: unable to handle page fault for address: 0000000000000488
  RIP: 0010:folio_memcg_list_lru_alloc+0x41/0xf0
  Call Trace:
   <TASK>
   shmem_get_folio_gfp+0x1cd/0x7c0
   shmem_write_begin+0x5d/0x100
   generic_perform_write+0x89/0x2a0
   shmem_file_write_iter+0x82/0x90
   vfs_write+0x256/0x410
   ksys_write+0x61/0xe0
   do_syscall_64+0x8d/0x460
   entry_SYSCALL_64_after_hwframe+0x76/0x7e

The faulting address is the offset of mem_cgroup->kmemcg_id,
dereferenced on a NULL memcg in memcg_list_lru_allocated():

  folio_memcg_list_lru_alloc()
    list_lru_memcg_aware()               <- true, only nokmem checked
    memcg = folio_memcg(folio)           <- NULL
    memcg_list_lru_allocated(memcg, lru)
      memcg->kmemcg_id                   <- NULL pointer dereference

Check mem_cgroup_disabled() in __list_lru_init() so that all
list_lrus fall back to plain per-node lists when the controller is
disabled, matching what the shrinker side already does
(shrinker_memcg_alloc() bails out on mem_cgroup_disabled()).  This
makes the mem_cgroup_disabled() check in callers unnecessary rather
than mandatory.

Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
---
Applies on top of mm-new.  No Fixes: tag since the commit that makes
the crash reachable ("mm: shmem: make unused huge shrinker memcg
aware") is only in mm-new; no stable backport is needed either.

Reproducer, on mm-new booted with cgroup_disable=memory
(CONFIG_MEMCG=y, CONFIG_TRANSPARENT_HUGEPAGE=y):

  # mount -t tmpfs -o huge=always tmpfs /mnt
  # echo x > /mnt/f

Without this patch the write oopses immediately as shown above: the
freshly allocated huge folio extends beyond i_size, so
shmem_get_folio_gfp() queues the inode via shmem_unused_huge_add()
-> folio_memcg_list_lru_alloc(), which dereferences the NULL
folio_memcg().

With this patch the same steps run cleanly: the lru falls back to
plain per-node lists and folio_memcg_list_lru_alloc() returns early.
From code inspection the rest of the shmem path handles the NULL
objcg fine (obj_cgroup_memcg() and obj_cgroup_put() are NULL-safe,
and list_lru_add() with a NULL memcg lands on the per-node list),
but I have not exercised the shrinker reclaim itself under
cgroup_disable=memory.

Discussion: https://lore.kernel.org/linux-mm/20260901115104.2944996-1-qinyuntan@linux.alibaba.com/

 mm/list_lru.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 36662d02ff963..f8be119351cca 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -671,7 +671,12 @@ int __list_lru_init(struct list_lru *lru, bool memcg_aware, struct shrinker *shr
 	else
 		lru->shrinker_id = -1;
 
-	if (mem_cgroup_kmem_disabled())
+	/*
+	 * With the memory controller disabled entirely, no object is ever
+	 * charged to a memcg, so collapse to plain per-node lists just
+	 * like under nokmem.
+	 */
+	if (mem_cgroup_disabled() || mem_cgroup_kmem_disabled())
 		memcg_aware = false;
 #endif
 
-- 
2.43.7