On Sat 10-06-17 10:49:01, Michal Hocko wrote: > On Fri 09-06-17 16:46:42, Michal Hocko wrote: > > On Fri 09-06-17 10:08:53, Johannes Weiner wrote: > > > On Thu, Jun 08, 2017 at 04:36:07PM +0200, Michal Hocko wrote: > > > > Does anybody see any problem with the patch or I can send it for the > > > > inclusion? > > > > > > > > On Fri 19-05-17 13:26:04, Michal Hocko wrote: > > > > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > > > > > > > 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 > > > > > 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. > > > > > > > > > > An allocation might fail also when the current task is the oom victim > > > > > and there are no memory reserves left and we should simply bail out > > > > > from the #PF rather than invoking out_of_memory. > > > > > > > > > > This all suggests that there is no legitimate reason to trigger > > > > > out_of_memory from pagefault_out_of_memory so drop it. Just to be sure > > > > > that no #PF path returns with VM_FAULT_OOM without allocation print a > > > > > warning that this is happening before we restart the #PF. > > > > > > > > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > > > > > I don't agree with this patch. > > > > > > The warning you replace the oom call with indicates that we never > > > expect a VM_FAULT_OOM to leak to this point. But should there be a > > > leak, it's infinitely better to tickle the OOM killer again - even if > > > that call is then fairly inaccurate and without alloc context - than > > > infinite re-invocations of the #PF when the VM_FAULT_OOM comes from a > > > context - existing or future - that isn't allowed to trigger the OOM. > > > > I disagree. Retrying the page fault while dropping all the locks > > on the way and still being in the killable context should be preferable > > to a system wide disruptive action like the OOM killer. > > And just to clarify a bit. The OOM killer should be invoked whenever > appropriate from the allocation context. If we decide to fail the > allocation in the PF path then we can safely roll back and retry the > whole PF. This has an advantage that any locks held while doing the > allocation will be released and that alone can help to make a further > progress. Moreover we can relax retry-for-ever _inside_ the allocator > semantic for the PF path and fail allocations when we cannot make > further progress even after we hit the OOM condition or we do stall for > too long. This would have a nice side effect that PF would be a killable > context from the page allocator POV. From the user space POV there is no > difference between retrying the PF and looping inside the allocator, > right? > > That being said, late just-in-case OOM killer invocation is not only > suboptimal it also disallows us to make further changes in that area. > > Or am I oversimplifying or missing something here? I am sorry to keep reviving this. I simply do not understand why the code actually make sense. If am missing something I would like to hear what it is. Then I will shut up (I promiss) ;) -- 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>
