On Tue, Aug 26, 2014 at 6:53 AM, Mel Gorman <mgorman@xxxxxxxx> wrote: > On Mon, Aug 25, 2014 at 04:48:52PM +1000, NeilBrown wrote: >> On Fri, 22 Aug 2014 18:49:31 -0400 Trond Myklebust >> <trond.myklebust@xxxxxxxxxxxxxxx> wrote: >> >> > Junxiao Bi reports seeing the following deadlock: >> > >> > @ crash> bt 1539 >> > @ PID: 1539 TASK: ffff88178f64a040 CPU: 1 COMMAND: "rpciod/1" >> > @ #0 [ffff88178f64d2c0] schedule at ffffffff8145833a >> > @ #1 [ffff88178f64d348] io_schedule at ffffffff8145842c >> > @ #2 [ffff88178f64d368] sync_page at ffffffff810d8161 >> > @ #3 [ffff88178f64d378] __wait_on_bit at ffffffff8145895b >> > @ #4 [ffff88178f64d3b8] wait_on_page_bit at ffffffff810d82fe >> > @ #5 [ffff88178f64d418] wait_on_page_writeback at ffffffff810e2a1a >> > @ #6 [ffff88178f64d438] shrink_page_list at ffffffff810e34e1 >> > @ #7 [ffff88178f64d588] shrink_list at ffffffff810e3dbe >> > @ #8 [ffff88178f64d6f8] shrink_zone at ffffffff810e425e >> > @ #9 [ffff88178f64d7b8] do_try_to_free_pages at ffffffff810e4978 >> > @ #10 [ffff88178f64d828] try_to_free_pages at ffffffff810e4c31 >> > @ #11 [ffff88178f64d8c8] __alloc_pages_nodemask at ffffffff810de370 >> >> This stack trace (from 2.6.32) cannot happen in mainline, though it took me a >> while to remember/discover exactly why. >> >> try_to_free_pages() creates a 'struct scan_control' with ->target_mem_cgroup >> set to NULL. >> shrink_page_list() checks ->target_mem_cgroup using global_reclaim() and if >> it is NULL, wait_on_page_writeback is *not* called. >> > > wait_on_page_writeback has a host of other damage associated with it which > is why we don't do it from reclaim any more. If the storage is very slow > then a process can be stalled by unrelated IO to slow storage. If the > storage is broken and the writeback can never complete then it causes other > issues. That kind of thing. > >> So we can only hit this deadlock if mem-cgroup limits are imposed on a >> process which is using NFS - which is quite possible but probably not common. >> >> The fact that a dead-lock can happen only when memcg limits are imposed seems >> very fragile. People aren't going to test that case much so there could well >> be other deadlock possibilities lurking. >> > > memcgs still can call wait_on_page_writeback and this is known to be a > hand-grenade to the memcg people but I've never heard of them trying to > tackle the problem. > >> Mel: might there be some other way we could get out of this deadlock? >> Could the wait_on_page_writeback() in shrink_page_list() be made a timed-out >> wait or something? Any other wait out of this deadlock other than setting >> PF_MEMALLOC_NOIO everywhere? >> > > I don't have the full thread as it was not cc'd to lkml so I don't know > what circumstances reached this deadlock in the first place. If this is > on 2.6.32 and the deadline cannot happen during reclaim in mainline then > why is mainline being patched? > > Do not alter wait_on_page_writeback() to timeout as it will blow > up spectacularly -- swap unuse races, data would not longer be synced > correctly to disk, sync IO would be flaky, stable page writes would be > fired out the window etc. Hi Mel, The above stack trace really is the entire deadlock: the rpciod work queue, which drives I/O on behalf of NFS, gets caught in a shrink_page_list() situation where it ends up waiting on page writeback. Boom.... Even if this can only happen for non-trivial memcg situations, then it still needs to be addressed: if rpciod blocks, then all NFS I/O will block and we can no longer write out the dirty pages. This is why we need a mainline fix. Cheers Trond -- Trond Myklebust Linux NFS client maintainer, PrimaryData trond.myklebust@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
