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SubjectRe: [PATCH] mm: fix hanging shrinker management on long do_shrink_slab

On 12/4/19 11:35 AM, Michal Hocko wrote:
> On Sat 30-11-19 00:45:41, Pavel Tikhomirov wrote:
>> We have a problem that shrinker_rwsem can be held for a long time for
>> read in shrink_slab, at the same time any process which is trying to
>> manage shrinkers hangs.
>> The shrinker_rwsem is taken in shrink_slab while traversing shrinker_list.
>> It tries to shrink something on nfs (hard) but nfs server is dead at
>> these moment already and rpc will never succeed. Generally any shrinker
>> can take significant time to do_shrink_slab, so it's a bad idea to hold
>> the list lock here.
> Yes, this is a known problem and people have already tried to address it
> in the past. Have you checked previous attempts? SRCU based one
> but I believe there were others (I only had this one in my notes).
> Please make sure to Cc Dave Chinner when posting a next version because
> he had some concerns about the change of the behavior.

The approach of the patch you are referencing is quiet different, it
will still hold global srcu_read_lock(&srcu) when diving in
do_shrink_slab and hanging nfs will still block all [un]register_shrinker.

>> We have a similar problem in shrink_slab_memcg, except that we are
>> traversing shrinker_map+shrinker_idr there.
>> The idea of the patch is to inc a refcount to the chosen shrinker so it
>> won't disappear and release shrinker_rwsem while we are in
>> do_shrink_slab, after that we will reacquire shrinker_rwsem, dec
>> the refcount and continue the traversal.
> The reference count part makes sense to me. RCU role needs a better
> explanation.

I have 2 rcu's in patch, 1-st is to protect shrinker_map same as it was
before in memcg_set_shrinker_bit, 2-nd is to protect shrinker struct in
put_shrinker from being freed, as unregister_shrinker can see refcnt==0
without actually going to schedule().

> Also do you have any reason to not use completion for
> the final step? Openconding essentially the same concept sounds a bit
> awkward to me.

Thanks for a good hint, from the first glance we can rework wait_event
part to wait_for_completion.

>> We also need a wait_queue so that unregister_shrinker can wait for the
>> refcnt to become zero. Only after these we can safely remove the
>> shrinker from list and idr, and free the shrinker.
> [...]
>> crash> bt ...
>> PID: 18739 TASK: ... CPU: 3 COMMAND: "bash"
>> #0 [...] __schedule at ...
>> #1 [...] schedule at ...
>> #2 [...] rpc_wait_bit_killable at ... [sunrpc]
>> #3 [...] __wait_on_bit at ...
>> #4 [...] out_of_line_wait_on_bit at ...
>> #5 [...] _nfs4_proc_delegreturn at ... [nfsv4]
>> #6 [...] nfs4_proc_delegreturn at ... [nfsv4]
>> #7 [...] nfs_do_return_delegation at ... [nfsv4]
>> #8 [...] nfs4_evict_inode at ... [nfsv4]
>> #9 [...] evict at ...
>> #10 [...] dispose_list at ...
>> #11 [...] prune_icache_sb at ...
>> #12 [...] super_cache_scan at ...
>> #13 [...] do_shrink_slab at ...
> Are NFS people aware of this? Because this is simply not acceptable
> behavior. Memory reclaim cannot be block indefinitely or for a long
> time. There must be a way to simply give up if the underlying inode
> cannot be reclaimed.

Sorry that I didn't cc nfs people from the begining.

> I still have to think about the proposed solution. It sounds a bit over
> complicated to me.

Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.
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