On Mon 18-07-16 19:00:57, David Rientjes wrote: > On Mon, 18 Jul 2016, Michal Hocko wrote: > > > David Rientjes was objecting that such an approach wouldn't help if the > > oom victim was blocked on a lock held by process doing mempool_alloc. This > > is very similar to other oom deadlock situations and we have oom_reaper > > to deal with them so it is reasonable to rely on the same mechanism > > rather inventing a different one which has negative side effects. > > > > Right, this causes oom livelock as described in the aforementioned thread: > the oom victim is waiting on a mutex that is held by a thread doing > mempool_alloc(). The backtrace you have provided: schedule schedule_timeout io_schedule_timeout mempool_alloc __split_and_process_bio dm_request generic_make_request submit_bio mpage_readpages ext4_readpages __do_page_cache_readahead ra_submit filemap_fault handle_mm_fault __do_page_fault do_page_fault page_fault is not PF_MEMALLOC context AFAICS so clearing __GFP_NOMEMALLOC for such a task will not help unless that task has TIF_MEMDIE. Could you provide a trace where the PF_MEMALLOC context holding a lock cannot make a forward progress? > The oom reaper is not guaranteed to free any memory, so > nothing on the system can allocate memory from the page allocator. Sure, there is no guarantee but as I've said earlier, 1) oom_reaper will allow to select another victim in many cases and 2) such a deadlock is no different from any other where the victim cannot continue because of another context blocking a lock while waiting for memory. Tweaking mempool allocator to potentially catch such a case in a different way doesn't sound right in principle, not to mention this is other dangerous side effects. > I think the better solution here is to allow mempool_alloc() users to set > __GFP_NOMEMALLOC if they are in a context which allows them to deplete > memory reserves. I am not really sure about that. I agree with Johannes [1] that this is bending mempool allocator into an undesirable direction because the point of the mempool is to have its own reliably reusable memory reserves. Now I am even not sure whether TIF_MEMDIE exception is a good way forward and a plain revert is more appropriate. Let's CC Johannes. The patch is [2]. [1] http://lkml.kernel.org/r/20160718151445.GB14604@xxxxxxxxxxx [2] http://lkml.kernel.org/r/1468831285-27242-1-git-send-email-mhocko@xxxxxxxxxx -- Michal Hocko SUSE Labs -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
