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Howlett" , Matthew Wilcox , Suren Baghdasaryan , Michal Hocko , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 79BF31C0003 X-Stat-Signature: mcdipfj88p4n8wuk4ftjsjjbse6qhgih X-Rspam-User: X-HE-Tag: 1674235079-882191 X-HE-Meta: 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 rK0dwYSL 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jan 20, 2023 at 9:08 AM Liam R. Howlett wrote: > > * Matthew Wilcox [230120 11:50]: > > On Fri, Jan 20, 2023 at 08:45:21AM -0800, Suren Baghdasaryan wrote: > > > On Fri, Jan 20, 2023 at 8:20 AM Suren Baghdasaryan wrote: > > > > > > > > On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko wrote: > > > > > > > > > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote: > > > > > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko wrote: > > > > > > > > > > > > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote: > > > > > > > > call_rcu() can take a long time when callback offloading is enabled. > > > > > > > > Its use in the vm_area_free can cause regressions in the exit path when > > > > > > > > multiple VMAs are being freed. To minimize that impact, place VMAs into > > > > > > > > a list and free them in groups using one call_rcu() call per group. > > > > > > > > > > > > > > After some more clarification I can understand how call_rcu might not be > > > > > > > super happy about thousands of callbacks to be invoked and I do agree > > > > > > > that this is not really optimal. > > > > > > > > > > > > > > On the other hand I do not like this solution much either. > > > > > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that > > > > > > > much with processes with a huge number of vmas either. It would still be > > > > > > > in housands of callbacks to be scheduled without a good reason. > > > > > > > > > > > > > > Instead, are there any other cases than remove_vma that need this > > > > > > > batching? We could easily just link all the vmas into linked list and > > > > > > > use a single call_rcu instead, no? This would both simplify the > > > > > > > implementation, remove the scaling issue as well and we do not have to > > > > > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1. > > > > > > > > > > > > Yes, I agree the solution is not stellar. I wanted something simple > > > > > > but this is probably too simple. OTOH keeping all dead vm_area_structs > > > > > > on the list without hooking up a shrinker (additional complexity) does > > > > > > not sound too appealing either. > > > > > > > > > > I suspect you have missed my idea. I do not really want to keep the list > > > > > around or any shrinker. It is dead simple. Collect all vmas in > > > > > remove_vma and then call_rcu the whole list at once after the whole list > > > > > (be it from exit_mmap or remove_mt). See? > > > > > > > > Yes, I understood your idea but keeping dead objects until the process > > > > exits even when the system is low on memory (no shrinkers attached) > > > > seems too wasteful. If we do this I would advocate for attaching a > > > > shrinker. > > > > > > Maybe even simpler, since we are hit with this VMA freeing flood > > > during exit_mmap (when all VMAs are destroyed), we pass a hint to > > > vm_area_free to batch the destruction and all other cases call > > > call_rcu()? I don't think there will be other cases of VMA destruction > > > floods. > > > > ... or have two different call_rcu functions; one for munmap() and > > one for exit. It'd be nice to use kmem_cache_free_bulk(). > > Do we even need a call_rcu on exit? At the point of freeing the VMAs we > have set the MMF_OOM_SKIP bit and unmapped the vmas under the read lock. > Once we have obtained the write lock again, I think it's safe to say we > can just go ahead and free the VMAs directly. I think that would be still racy if the page fault handler found that VMA under read-RCU protection but did not lock it yet (no locks are held yet). If it's preempted, the VMA can be freed and destroyed from under it without RCU grace period. > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com. >