Michal Hocko wrote: > On Sat 20-05-17 00:22:30, Tetsuo Handa wrote: > > Michal Hocko wrote: > > > On Fri 19-05-17 22:02:44, Tetsuo Handa wrote: > > > > Michal Hocko wrote: > > > > > Any allocation failure during the #PF path will return with VM_FAULT_OOM > > > > > which in turn results in pagefault_out_of_memory. This can happen for > > > > > 2 different reasons. a) Memcg is out of memory and we rely on > > > > > mem_cgroup_oom_synchronize to perform the memcg OOM handling or b) > > > > > normal allocation fails. > > > > > > > > > > The later is quite problematic because allocation paths already trigger > > > > > out_of_memory and the page allocator tries really hard to not fail > > > > > > > > We made many memory allocation requests from page fault path (e.g. XFS) > > > > __GFP_FS some time ago, didn't we? But if I recall correctly (I couldn't > > > > find the message), there are some allocation requests from page fault path > > > > which cannot use __GFP_FS. Then, not all allocation requests can call > > > > oom_kill_process() and reaching pagefault_out_of_memory() will be > > > > inevitable. > > > > > > Even if such an allocation fail without the OOM killer then we simply > > > retry the PF and will do that the same way how we keep retrying the > > > allocation inside the page allocator. So how is this any different? > > > > You are trying to remove out_of_memory() from pagefault_out_of_memory() > > by this patch. But you also want to make !__GFP_FS allocations not to > > keep retrying inside the page allocator in future kernels, don't you? > > I would _love_ to but I am much less optimistic this is achiveable > > > Then, a thread which need to allocate memory from page fault path but > > cannot call oom_kill_process() will spin forever (unless somebody else > > calls oom_kill_process() via a __GFP_FS allocation request). I consider > > that introducing such possibility is a problem. > > What I am trying to say is that this is already happening. The > difference with the VM_FAULT_OOM would only be that the whole PF path > would be unwinded back to the PF, all locks dropped and then the PF > retries so in principle this would be safer. I can't understand why the PF retries helps. If the PF has to be retried due to failing to allocate memory, situation will not improve until memory is allocated. And I don't like an assumption that somebody else calls oom_kill_process() via a __GFP_FS allocation request so that the PF will succeed without calling oom_kill_process(). > > > > > > allocations. Anyway, if the OOM killer has been already invoked there > > > > > is no reason to invoke it again from the #PF path. Especially when the > > > > > OOM condition might be gone by that time and we have no way to find out > > > > > other than allocate. > > > > > > > > > > Moreover if the allocation failed and the OOM killer hasn't been > > > > > invoked then we are unlikely to do the right thing from the #PF context > > > > > because we have already lost the allocation context and restictions and > > > > > therefore might oom kill a task from a different NUMA domain. > > > > > > > > If we carry a flag via task_struct that indicates whether it is an memory > > > > allocation request from page fault and allocation failure is not acceptable, > > > > we can call out_of_memory() from page allocator path. > > > > > > I do not understand > > > > We need to allocate memory from page fault path in order to avoid spinning forever > > (unless somebody else calls oom_kill_process() via a __GFP_FS allocation request), > > doesn't it? Then, memory allocation requests from page fault path can pass flags > > like __GFP_NOFAIL | __GFP_KILLABLE because retrying the page fault without > > allocating memory is pointless. I called such flags as carry a flag via task_struct. > > > > > > By the way, can page fault occur after reaching do_exit()? When a thread > > > > reached do_exit(), fatal_signal_pending(current) becomes false, doesn't it? > > > > > > yes fatal_signal_pending will be false at the time and I believe we can > > > perform a page fault past that moment and go via allocation path which would > > > trigger the OOM or give this task access to reserves but it is more > > > likely that the oom reaper will push to kill another task by that time > > > if the situation didn't get resolved. Or did I miss your concern? > > > > How checking fatal_signal_pending() here helps? > > It just skips the warning because we know that we would handle the > signal before retrying the page fault and go to exit path. Those that do > not have such a signal should warn just that we know that such a > situation happens. With the current allocator semantic it shouldn't > > > It only suppresses printk(). > > If current thread needs to allocate memory because not all allocation requests > > can call oom_kill_process(), doing printk() is not the right thing to do. > > Allocate memory by some means (e.g. __GFP_NOFAIL | __GFP_KILLABLE) will be > > the right thing to do. > > Why would looping inside an allocator with a restricted context be any > better than retrying the whole thing? I'm not suggesting you to loop inside an allocator nor retry the whole thing. I'm suggesting you to avoid returning VM_FAULT_OOM by making allocations succeed (by e.g. calling oom_kill_process()) regardless of restricted context if you want to remove out_of_memory() from pagefault_out_of_memory(), for situation will not improve until memory is allocated (e.g. somebody else calls oom_kill_process() via a __GFP_FS allocation request). -- 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>
