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

[Shakeel Butt <shakeelb@google.com>]

On Mon, Dec 2, 2019 at 8:37 AM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>
>
> On 11/30/19 12:45 AM, 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.
> >
> > 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.
> >
> > 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.
> >
> > I've reproduced the nfs hang in do_shrink_slab with the patch applied on
> > ms kernel, all other mount/unmount pass fine without any hang.
> >
> > Here is a reproduction on kernel without patch:
> >
> > 1) Setup nfs on server node with some files in it (e.g. 200)
> >
> > [server]# cat /etc/exports
> > /vz/nfs2 *(ro,no_root_squash,no_subtree_check,async)
> >
> > 2) Hard mount it on client node
> >
> > [client]# mount -ohard 10.94.3.40:/vz/nfs2 /mnt
> >
> > 3) Open some (e.g. 200) files on the mount
> >
> > [client]# for i in $(find /mnt/ -type f | head -n 200); \
> >   do setsid sleep 1000 &>/dev/null <$i & done
> >
> > 4) Kill all openers
> >
> > [client]# killall sleep -9
> >
> > 5) Put your network cable out on client node
> >
> > 6) Drop caches on the client, it will hang on nfs while holding
> >   shrinker_rwsem lock for read
> >
> > [client]# echo 3 > /proc/sys/vm/drop_caches
> >
> >   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 ...
> >   #14 [...] shrink_slab at ...
> >   #15 [...] drop_slab_node at ...
> >   #16 [...] drop_slab at ...
> >   #17 [...] drop_caches_sysctl_handler at ...
> >   #18 [...] proc_sys_call_handler at ...
> >   #19 [...] vfs_write at ...
> >   #20 [...] ksys_write at ...
> >   #21 [...] do_syscall_64 at ...
> >   #22 [...] entry_SYSCALL_64_after_hwframe at ...
> >
> > 7) All other mount/umount activity now hangs with no luck to take
> >   shrinker_rwsem for write.
> >
> > [client]# mount -t tmpfs tmpfs /tmp
> >
> >   crash> bt ...
> >   PID: 5464   TASK: ...  CPU: 3   COMMAND: "mount"
> >    #0 [...] __schedule at ...
> >    #1 [...] schedule at ...
> >    #2 [...] rwsem_down_write_slowpath at ...
> >    #3 [...] prealloc_shrinker at ...
> >    #4 [...] alloc_super at ...
> >    #5 [...] sget at ...
> >    #6 [...] mount_nodev at ...
> >    #7 [...] legacy_get_tree at ...
> >    #8 [...] vfs_get_tree at ...
> >    #9 [...] do_mount at ...
> >   #10 [...] ksys_mount at ...
> >   #11 [...] __x64_sys_mount at ...
> >   #12 [...] do_syscall_64 at ...
> >   #13 [...] entry_SYSCALL_64_after_hwframe at ...
> >
>
>
> I don't think this patch solves the problem, it only fixes one minor symptom of it.
> The actual problem here the reclaim hang in the nfs.
> It means that any process, including kswapd, may go into nfs inode reclaim and stuck there.
>
> Even mount() itself has GFP_KERNEL allocations in its path, so it still might stuck there even with your patch.
>
> I think this should be handled on nfs/vfs level by making  inode eviction during reclaim more asynchronous.

Though I agree that we should be fixing shrinkers to not get stuck
(and be more async), I still think the problem this patch is solving
is worth fixing. On machines running multiple workloads, one job stuck
in slab shrinker and blocking all other unrelated jobs wanting
shrinker_rwsem, breaks the isolation and causes DoS.

Shakeel