* Matthew Wilcox [off-list ref] [230120 11:50]:

On Fri, Jan 20, 2023 at 08:45:21AM -0800, Suren Baghdasaryan wrote:

quoted

On Fri, Jan 20, 2023 at 8:20 AM Suren Baghdasaryan [off-list ref] wrote:

quoted

On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko [off-list ref] wrote:

quoted

On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote:

quoted

On Thu, Jan 19, 2023 at 4:59 AM Michal Hocko [off-list ref] wrote:

quoted

On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote:

quoted

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.