On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <wi...@infradead.org> wrote: > > On Fri, Jan 20, 2023 at 09:17:46AM -0800, Suren Baghdasaryan wrote: > > On Fri, Jan 20, 2023 at 9:08 AM Liam R. Howlett <liam.howl...@oracle.com> > > wrote: > > > > > > * Matthew Wilcox <wi...@infradead.org> [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 > > > > > <sur...@google.com> wrote: > > > > > > > > > > > > On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko <mho...@suse.com> > > > > > > wrote: > > > > > > > > > > > > > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote: > > > > > > > > On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko <mho...@suse.com> > > > > > > > > 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. > > The page fault handler (or whatever other reader -- ptrace, proc, etc) > should have a refcount on the mm_struct, so we can't be in this path > trying to free VMAs. Right?
Hmm. That sounds right. I checked process_mrelease() as well, which operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps VMAs without freeing them, so we are still good. Michal, do you agree this is ok? lock_vma_under_rcu() receives mm as a parameter, so I guess it's implied that the caller should either mmget() it or operate on current->mm, so no need to document this requirement?
