Re: [PATCH 2/5] mm, memcg: narrow the scope of percpu_charge_mutex

Miaohe Lin
2021-08-05T01:44:40Z

On 2021/8/4 16:20, Michal Hocko wrote:
> On Tue 03-08-21 10:15:36, Johannes Weiner wrote:
> [...]
>> git history shows we tried to remove it once:
>>
>> commit 8521fc50d433507a7cdc96bec280f9e5888a54cc
>> Author: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
>> Date: Tue Jul 26 16:08:29 2011 -0700
>>
>> memcg: get rid of percpu_charge_mutex lock
>>
>> but it turned out that the lock did in fact protect a data structure:
>> the stock itself. Specifically stock->cached:
>>
>> commit 9f50fad65b87a8776ae989ca059ad6c17925dfc3
>> Author: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
>> Date: Tue Aug 9 11:56:26 2011 +0200
>>
>> Revert "memcg: get rid of percpu_charge_mutex lock"
>>
>> This reverts commit 8521fc50d433507a7cdc96bec280f9e5888a54cc.
>>
>> The patch incorrectly assumes that using atomic FLUSHING_CACHED_CHARGE
>> bit operations is sufficient but that is not true. Johannes Weiner has
>> reported a crash during parallel memory cgroup removal:
>>
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
>> IP: [<ffffffff81083b70>] css_is_ancestor+0x20/0x70
>> Oops: 0000 [#1] PREEMPT SMP
>> Pid: 19677, comm: rmdir Tainted: G W 3.0.0-mm1-00188-gf38d32b #35 ECS MCP61M-M3/MCP61M-M3
>> RIP: 0010:[<ffffffff81083b70>] css_is_ancestor+0x20/0x70
>> RSP: 0018:ffff880077b09c88 EFLAGS: 00010202
>> Process rmdir (pid: 19677, threadinfo ffff880077b08000, task ffff8800781bb310)
>> Call Trace:
>> [<ffffffff810feba3>] mem_cgroup_same_or_subtree+0x33/0x40
>> [<ffffffff810feccf>] drain_all_stock+0x11f/0x170
>> [<ffffffff81103211>] mem_cgroup_force_empty+0x231/0x6d0
>> [<ffffffff811036c4>] mem_cgroup_pre_destroy+0x14/0x20
>> [<ffffffff81080559>] cgroup_rmdir+0xb9/0x500
>> [<ffffffff81114d26>] vfs_rmdir+0x86/0xe0
>> [<ffffffff81114e7b>] do_rmdir+0xfb/0x110
>> [<ffffffff81114ea6>] sys_rmdir+0x16/0x20
>> [<ffffffff8154d76b>] system_call_fastpath+0x16/0x1b
>>
>> We are crashing because we try to dereference cached memcg when we are
>> checking whether we should wait for draining on the cache. The cache is
>> already cleaned up, though.
>>
>> There is also a theoretical chance that the cached memcg gets freed
>> between we test for the FLUSHING_CACHED_CHARGE and dereference it in
>> mem_cgroup_same_or_subtree:
>>
>> CPU0 CPU1 CPU2
>> mem=stock->cached
>> stock->cached=NULL
>> clear_bit
>> test_and_set_bit
>> test_bit() ...
>> <preempted> mem_cgroup_destroy
>> use after free
>>
>> The percpu_charge_mutex protected from this race because sync draining
>> is exclusive.
>>
>> It is safer to revert now and come up with a more parallel
>> implementation later.
>>
>> I didn't remember this one at all!
>
> Me neither. Thanks for looking that up!
>
>> However, when you look at the codebase from back then, there was no
>> rcu-protection for memcg lifetime, and drain_stock() didn't double
>> check stock->cached inside the work. Hence the crash during a race.
>>
>> The drain code is different now: drain_local_stock() disables IRQs
>> which holds up rcu, and then calls drain_stock() and drain_obj_stock()
>> which both check stock->cached one more time before the deref.
>>
>> With workqueue managing concurrency, and rcu ensuring memcg lifetime
>> during the drain, this lock indeed seems unnecessary now.
>>
>> Unless I'm missing something, it should just be removed instead.
>
> I do not think you are missing anything. We can drop the lock and
> simplify the code. The above information would be great to have in the
> changelog.
>

Am I supposed to revert this with the above information in the changelog and add
Suggested-by for both of you?

Many thanks.

> Thanks!
>