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@xxxxxxx> >> 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@xxxxxxx> >> 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! >
