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SubjectRe: [PATCH] mm: fix hanging shrinker management on long do_shrink_slab
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On 30.11.2019 00:45, 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 ...
>
> That is on almost clean and almost mainstream Fedora kernel:
>
> [client]# uname -a
> Linux snorch 5.3.8-200.snorch.fc30.x86_64 #1 SMP Mon Nov 11 16:01:15 MSK
> 2019 x86_64 x86_64 x86_64 GNU/Linux
>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

The approach looks OK for me, and despite it does not solve the problem
completely, it improves the isolation. Also, we may use this to finally
make shrink_slab() iterations lockless (or you may just send couple of
patches on top of that :).

Small cosmetic note. I'd removed comments in prealloc_shrinker() and
free_preallocated_shrinker() since they just describe obvious actions,
and we do not need to comment every line we do. But a small comment
is needed about that synchronize_rcu() is to make wake_up on stable
memory.

> ---
> include/linux/memcontrol.h | 6 +++
> include/linux/shrinker.h | 6 +++
> mm/memcontrol.c | 16 ++++++
> mm/vmscan.c | 107 ++++++++++++++++++++++++++++++++++++-
> 4 files changed, 134 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index ae703ea3ef48..3717b94b6aa5 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1348,6 +1348,8 @@ extern int memcg_expand_shrinker_maps(int new_id);
>
> extern void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> int nid, int shrinker_id);
> +extern void memcg_clear_shrinker_bit(struct mem_cgroup *memcg,
> + int nid, int shrinker_id);
> #else
> #define mem_cgroup_sockets_enabled 0
> static inline void mem_cgroup_sk_alloc(struct sock *sk) { };
> @@ -1361,6 +1363,10 @@ static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
> int nid, int shrinker_id)
> {
> }
> +static inline void memcg_clear_shrinker_bit(struct mem_cgroup *memcg,
> + int nid, int shrinker_id)
> +{
> +}
> #endif
>
> struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep);
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 0f80123650e2..dd3bb43ed58d 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -2,6 +2,9 @@
> #ifndef _LINUX_SHRINKER_H
> #define _LINUX_SHRINKER_H
>
> +#include <linux/refcount.h>
> +#include <linux/wait.h>
> +
> /*
> * This struct is used to pass information from page reclaim to the shrinkers.
> * We consolidate the values for easier extention later.
> @@ -75,6 +78,9 @@ struct shrinker {
> #endif
> /* objs pending delete, per node */
> atomic_long_t *nr_deferred;
> +
> + refcount_t refcnt;
> + wait_queue_head_t wq;
> };
> #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 01f3f8b665e9..81f45124feb7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -442,6 +442,22 @@ void memcg_set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
> }
> }
>
> +void memcg_clear_shrinker_bit(struct mem_cgroup *memcg,
> + int nid, int shrinker_id)
> +{
> + struct memcg_shrinker_map *map;
> +
> + /*
> + * The map for refcounted memcg can only be freed in
> + * memcg_free_shrinker_map_rcu so we can safely protect
> + * map with rcu_read_lock.
> + */
> + rcu_read_lock();
> + map = rcu_dereference(memcg->nodeinfo[nid]->shrinker_map);
> + clear_bit(shrinker_id, map->map);
> + rcu_read_unlock();
> +}
> +
> /**
> * mem_cgroup_css_from_page - css of the memcg associated with a page
> * @page: page of interest
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ee4eecc7e1c2..59e46d65e902 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -393,6 +393,13 @@ int prealloc_shrinker(struct shrinker *shrinker)
> if (!shrinker->nr_deferred)
> return -ENOMEM;
>
> + /*
> + * Shrinker is not yet visible through shrinker_idr or shrinker_list,
> + * so no locks required for initialization.
> + */
> + refcount_set(&shrinker->refcnt, 1);
> + init_waitqueue_head(&shrinker->wq);
> +
> if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> if (prealloc_memcg_shrinker(shrinker))
> goto free_deferred;
> @@ -411,6 +418,9 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
> if (!shrinker->nr_deferred)
> return;
>
> + /* The shrinker shouldn't be used at these point. */
> + WARN_ON(!refcount_dec_and_test(&shrinker->refcnt));
> +
> if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> unregister_memcg_shrinker(shrinker);
>
> @@ -447,6 +457,15 @@ void unregister_shrinker(struct shrinker *shrinker)
> {
> if (!shrinker->nr_deferred)
> return;
> +
> + /*
> + * If refcnt is not zero we need to wait these shrinker to finish all
> + * it's do_shrink_slab() calls.
> + */
> + if (!refcount_dec_and_test(&shrinker->refcnt))
> + wait_event(shrinker->wq,
> + (refcount_read(&shrinker->refcnt) == 0));
> +
> if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> unregister_memcg_shrinker(shrinker);
> down_write(&shrinker_rwsem);
> @@ -454,6 +473,9 @@ void unregister_shrinker(struct shrinker *shrinker)
> up_write(&shrinker_rwsem);
> kfree(shrinker->nr_deferred);
> shrinker->nr_deferred = NULL;
> +
> + /* Pairs with rcu_read_lock in put_shrinker() */
> + synchronize_rcu();
> }
> EXPORT_SYMBOL(unregister_shrinker);
>
> @@ -589,6 +611,42 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> return freed;
> }
>
> +struct shrinker *get_shrinker(struct shrinker *shrinker)
> +{
> + /*
> + * Pairs with refcnt dec in unregister_shrinker(), if refcnt is zero
> + * these shrinker is already in the middle of unregister_shrinker() and
> + * we can't use it.
> + */
> + if (!refcount_inc_not_zero(&shrinker->refcnt))
> + shrinker = NULL;
> + return shrinker;
> +}
> +
> +void put_shrinker(struct shrinker *shrinker)
> +{
> + /*
> + * The rcu_read_lock pairs with synchronize_rcu() in
> + * unregister_shrinker(), so that the shrinker is not freed
> + * before the wake_up.
> + */
> + rcu_read_lock();
> + if (!refcount_dec_and_test(&shrinker->refcnt)) {
> + /*
> + * Pairs with smp_mb in
> + * wait_event()->prepare_to_wait()
> + */
> + smp_mb();
> + /*
> + * If refcnt becomes zero, we already have an
> + * unregister_shrinker() waiting for us to finish.
> + */
> + if (waitqueue_active(&shrinker->wq))
> + wake_up(&shrinker->wq);
> + }
> + rcu_read_unlock();
> +}
> +
> #ifdef CONFIG_MEMCG
> static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> struct mem_cgroup *memcg, int priority)
> @@ -628,9 +686,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> !(shrinker->flags & SHRINKER_NONSLAB))
> continue;
>
> + /*
> + * Take a refcnt on a shrinker so that it can't be freed or
> + * removed from shrinker_idr (and shrinker_list). These way we
> + * can safely release shrinker_rwsem.
> + *
> + * We need to release shrinker_rwsem here as do_shrink_slab can
> + * take too much time to finish (e.g. on nfs). And holding
> + * global shrinker_rwsem can block registring and unregistring
> + * of shrinkers.
> + */
> + if (!get_shrinker(shrinker))
> + continue;
> + up_read(&shrinker_rwsem);
> +
> ret = do_shrink_slab(&sc, shrinker, priority);
> if (ret == SHRINK_EMPTY) {
> - clear_bit(i, map->map);
> + memcg_clear_shrinker_bit(memcg, nid, i);
> /*
> * After the shrinker reported that it had no objects to
> * free, but before we cleared the corresponding bit in
> @@ -655,6 +727,22 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> }
> freed += ret;
>
> + /*
> + * Re-aquire shrinker_rwsem, put refcount and reload
> + * shrinker_map as it can be modified in
> + * memcg_expand_one_shrinker_map if new shrinkers
> + * were registred in the meanwhile.
> + */
> + if (!down_read_trylock(&shrinker_rwsem)) {
> + freed = freed ? : 1;
> + put_shrinker(shrinker);
> + return freed;
> + }
> + put_shrinker(shrinker);
> + map = rcu_dereference_protected(
> + memcg->nodeinfo[nid]->shrinker_map,
> + true);
> +
> if (rwsem_is_contended(&shrinker_rwsem)) {
> freed = freed ? : 1;
> break;
> @@ -719,10 +807,27 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> .memcg = memcg,
> };
>
> + /* See comment in shrink_slab_memcg. */
> + if (!get_shrinker(shrinker))
> + continue;
> + up_read(&shrinker_rwsem);
> +
> ret = do_shrink_slab(&sc, shrinker, priority);
> if (ret == SHRINK_EMPTY)
> ret = 0;
> freed += ret;
> +
> + /*
> + * We can safely continue traverse of the shrinker_list as
> + * our shrinker is still on the list due to refcount.
> + */
> + if (!down_read_trylock(&shrinker_rwsem)) {
> + freed = freed ? : 1;
> + put_shrinker(shrinker);
> + goto out;
> + }
> + put_shrinker(shrinker);
> +
> /*
> * Bail out if someone want to register a new shrinker to
> * prevent the regsitration from being stalled for long periods
>

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