On Tue 05-02-13 10:09:57, Greg Thelen wrote: > On Tue, Feb 05 2013, Michal Hocko wrote: > > > On Tue 05-02-13 08:48:23, Greg Thelen wrote: > >> On Tue, Feb 05 2013, Michal Hocko wrote: > >> > >> > On Tue 05-02-13 15:49:47, azurIt wrote: > >> > [...] > >> >> Just to be sure - am i supposed to apply this two patches? > >> >> http://watchdog.sk/lkml/patches/ > >> > > >> > 5-memcg-fix-1.patch is not complete. It doesn't contain the folloup I > >> > mentioned in a follow up email. Here is the full patch: > >> > --- > >> > From f2bf8437d5b9bb38a95a432bf39f32c584955171 Mon Sep 17 00:00:00 2001 > >> > From: Michal Hocko <mhocko@xxxxxxx> > >> > Date: Mon, 26 Nov 2012 11:47:57 +0100 > >> > Subject: [PATCH] memcg: do not trigger OOM from add_to_page_cache_locked > >> > > >> > memcg oom killer might deadlock if the process which falls down to > >> > mem_cgroup_handle_oom holds a lock which prevents other task to > >> > terminate because it is blocked on the very same lock. > >> > This can happen when a write system call needs to allocate a page but > >> > the allocation hits the memcg hard limit and there is nothing to reclaim > >> > (e.g. there is no swap or swap limit is hit as well and all cache pages > >> > have been reclaimed already) and the process selected by memcg OOM > >> > killer is blocked on i_mutex on the same inode (e.g. truncate it). > >> > > >> > Process A > >> > [<ffffffff811109b8>] do_truncate+0x58/0xa0 # takes i_mutex > >> > [<ffffffff81121c90>] do_last+0x250/0xa30 > >> > [<ffffffff81122547>] path_openat+0xd7/0x440 > >> > [<ffffffff811229c9>] do_filp_open+0x49/0xa0 > >> > [<ffffffff8110f7d6>] do_sys_open+0x106/0x240 > >> > [<ffffffff8110f950>] sys_open+0x20/0x30 > >> > [<ffffffff815b5926>] system_call_fastpath+0x18/0x1d > >> > [<ffffffffffffffff>] 0xffffffffffffffff > >> > > >> > Process B > >> > [<ffffffff8110a9c1>] mem_cgroup_handle_oom+0x241/0x3b0 > >> > [<ffffffff8110b5ab>] T.1146+0x5ab/0x5c0 > >> > [<ffffffff8110c22e>] mem_cgroup_cache_charge+0xbe/0xe0 > >> > [<ffffffff810ca28c>] add_to_page_cache_locked+0x4c/0x140 > >> > [<ffffffff810ca3a2>] add_to_page_cache_lru+0x22/0x50 > >> > [<ffffffff810ca45b>] grab_cache_page_write_begin+0x8b/0xe0 > >> > [<ffffffff81193a18>] ext3_write_begin+0x88/0x270 > >> > [<ffffffff810c8fc6>] generic_file_buffered_write+0x116/0x290 > >> > [<ffffffff810cb3cc>] __generic_file_aio_write+0x27c/0x480 > >> > [<ffffffff810cb646>] generic_file_aio_write+0x76/0xf0 # takes ->i_mutex > >> > [<ffffffff8111156a>] do_sync_write+0xea/0x130 > >> > [<ffffffff81112183>] vfs_write+0xf3/0x1f0 > >> > [<ffffffff81112381>] sys_write+0x51/0x90 > >> > [<ffffffff815b5926>] system_call_fastpath+0x18/0x1d > >> > [<ffffffffffffffff>] 0xffffffffffffffff > >> > >> It looks like grab_cache_page_write_begin() passes __GFP_FS into > >> __page_cache_alloc() and mem_cgroup_cache_charge(). Which makes me > >> think that this deadlock is also possible in the page allocator even > >> before getting to add_to_page_cache_lru. no? > > > > I am not that familiar with VFS but i_mutex is a high level lock AFAIR > > and it shouldn't be called from the pageout path so __page_cache_alloc > > should be safe. > > I wasn't clear, sorry. My concern is not that pageout() grabs i_mutex. > My concern is that __page_cache_alloc() will invoke the oom killer and > select a victim which wants i_mutex. This victim will deadlock because > the oom killer caller already holds i_mutex. That would be true for the memcg oom because that one is blocking but the global oom just puts the allocator into sleep for a while and then the allocator should back off eventually (unless this is NOFAIL allocation). I would need to look closer whether this is really the case - I haven't seen that allocator code path for a while... > The wild accusation I am making is that anyone who invokes the oom > killer and waits on the victim to die is essentially grabbing all of > the locks that any of the oom killer victims may grab (e.g. i_mutex). True. > To avoid deadlock the oom killer can only be called is while holding > no locks that the oom victim demands. I think some locks are grabbed > in a way that allows the lock request to fail if the task has a fatal > signal pending, so they are safe. But any locks acquisitions that > cannot fail (e.g. mutex_lock) will deadlock with the oom killing > process. So the oom killing process cannot hold any such locks which > the victim will attempt to grab. Hopefully I'm missing something. Agreed. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>