On Sun, Jan 15, 2023 at 6:47 AM msizanoen1 msizanoen@qtmlabs.xyz wrote:
In some cases, memory cgroup migration can be initiated by userspace right after a process was created and right before `lru_gen_add_mm()` is called (e.g. by some program watching a cgroup and moving away any processes it detects[1]), which results in the following sequence of WARNs followed by an Oops as the kernel attempts to perform a `lru_gen_add_mm()` twice on the same `mm`:
...
Fix this by simply leaving the lru_gen alone if it has not been associated with a memcg yet, as it should eventually be assigned to the right cgroup anyway.
v2: Added stable cc tags
Signed-off-by: N/A (patch should not be copyrightable) Cc: stable@vger.kernel.org
Thanks for the fix. Cc'ing stable is the right thing to do. The commit message and the comment styles could be easily adjusted to align with the guidelines.
I don't think the N/A is acceptible though. I fully respect it if you wish to remain anonymous -- I can send a similar fix crediting you as the "anonymous user msizanoen@qtmlabs.xyz" who reported this bug.
A bit of background on how I broke it: an old version I have on 4.15 calls lru_gen_add_mm() before cgroup_post_fork(), which excludes cgroup migrations by cgroup_threadgroup_rwsem. When I rebased it, I made lru_gen_add_mm() depend on task_lock for the synchronization with cgroup migrations -- the decoupling seemed (still seems) to make it less complicated -- but this is not safe unless we have the check below.
mm/vmscan.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c index bd6637fcd8f9..0cac40e7484c 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3323,13 +3323,19 @@ void lru_gen_migrate_mm(struct mm_struct *mm) if (mem_cgroup_disabled()) return;
/* This could happen if cgroup migration is invoked before the process* lru_gen is associated with a memcg (e.g. during process creation).* Simply ignore it in this case as the lru_gen will get assigned the* right cgroup later. */if (!mm->lru_gen.memcg)return;rcu_read_lock(); memcg = mem_cgroup_from_task(task); rcu_read_unlock(); if (memcg == mm->lru_gen.memcg) return;
VM_WARN_ON_ONCE(!mm->lru_gen.memcg); VM_WARN_ON_ONCE(list_empty(&mm->lru_gen.list)); lru_gen_del_mm(mm);