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    Subject[PATCH 5.3 172/180] mm: memcg/slab: wait for !root kmem_cache refcnt killing on root kmem_cache destruction
    Date
    From: Roman Gushchin <guro@fb.com>

    commit a264df74df38855096393447f1b8f386069a94b9 upstream.

    Christian reported a warning like the following obtained during running
    some KVM-related tests on s390:

    WARNING: CPU: 8 PID: 208 at lib/percpu-refcount.c:108 percpu_ref_exit+0x50/0x58
    Modules linked in: kvm(-) xt_CHECKSUM xt_MASQUERADE bonding xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_na>
    CPU: 8 PID: 208 Comm: kworker/8:1 Not tainted 5.2.0+ #66
    Hardware name: IBM 2964 NC9 712 (LPAR)
    Workqueue: events sysfs_slab_remove_workfn
    Krnl PSW : 0704e00180000000 0000001529746850 (percpu_ref_exit+0x50/0x58)
    R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
    Krnl GPRS: 00000000ffff8808 0000001529746740 000003f4e30e8e18 0036008100000000
    0000001f00000000 0035008100000000 0000001fb3573ab8 0000000000000000
    0000001fbdb6de00 0000000000000000 0000001529f01328 0000001fb3573b00
    0000001fbb27e000 0000001fbdb69300 000003e009263d00 000003e009263cd0
    Krnl Code: 0000001529746842: f0a0000407fe srp 4(11,%r0),2046,0
    0000001529746848: 47000700 bc 0,1792
    #000000152974684c: a7f40001 brc 15,152974684e
    >0000001529746850: a7f4fff2 brc 15,1529746834
    0000001529746854: 0707 bcr 0,%r7
    0000001529746856: 0707 bcr 0,%r7
    0000001529746858: eb8ff0580024 stmg %r8,%r15,88(%r15)
    000000152974685e: a738ffff lhi %r3,-1
    Call Trace:
    ([<000003e009263d00>] 0x3e009263d00)
    [<00000015293252ea>] slab_kmem_cache_release+0x3a/0x70
    [<0000001529b04882>] kobject_put+0xaa/0xe8
    [<000000152918cf28>] process_one_work+0x1e8/0x428
    [<000000152918d1b0>] worker_thread+0x48/0x460
    [<00000015291942c6>] kthread+0x126/0x160
    [<0000001529b22344>] ret_from_fork+0x28/0x30
    [<0000001529b2234c>] kernel_thread_starter+0x0/0x10
    Last Breaking-Event-Address:
    [<000000152974684c>] percpu_ref_exit+0x4c/0x58
    ---[ end trace b035e7da5788eb09 ]---

    The problem occurs because kmem_cache_destroy() is called immediately
    after deleting of a memcg, so it races with the memcg kmem_cache
    deactivation.

    flush_memcg_workqueue() at the beginning of kmem_cache_destroy() is
    supposed to guarantee that all deactivation processes are finished, but
    failed to do so. It waits for an rcu grace period, after which all
    children kmem_caches should be deactivated. During the deactivation
    percpu_ref_kill() is called for non root kmem_cache refcounters, but it
    requires yet another rcu grace period to finish the transition to the
    atomic (dead) state.

    So in a rare case when not all children kmem_caches are destroyed at the
    moment when the root kmem_cache is about to be gone, we need to wait
    another rcu grace period before destroying the root kmem_cache.

    This issue can be triggered only with dynamically created kmem_caches
    which are used with memcg accounting. In this case per-memcg child
    kmem_caches are created. They are deactivated from the cgroup removing
    path. If the destruction of the root kmem_cache is racing with the
    removal of the cgroup (both are quite complicated multi-stage
    processes), the described issue can occur. The only known way to
    trigger it in the real life, is to unload some kernel module which
    creates a dedicated kmem_cache, used from different memory cgroups with
    GFP_ACCOUNT flag. If the unloading happens immediately after calling
    rmdir on the corresponding cgroup, there is some chance to trigger the
    issue.

    Link: http://lkml.kernel.org/r/20191129025011.3076017-1-guro@fb.com
    Fixes: f0a3a24b532d ("mm: memcg/slab: rework non-root kmem_cache lifecycle management")
    Signed-off-by: Roman Gushchin <guro@fb.com>
    Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Reviewed-by: Shakeel Butt <shakeelb@google.com>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/slab_common.c | 12 ++++++++++++
    1 file changed, 12 insertions(+)

    --- a/mm/slab_common.c
    +++ b/mm/slab_common.c
    @@ -904,6 +904,18 @@ static void flush_memcg_workqueue(struct
    * previous workitems on workqueue are processed.
    */
    flush_workqueue(memcg_kmem_cache_wq);
    +
    + /*
    + * If we're racing with children kmem_cache deactivation, it might
    + * take another rcu grace period to complete their destruction.
    + * At this moment the corresponding percpu_ref_kill() call should be
    + * done, but it might take another rcu grace period to complete
    + * switching to the atomic mode.
    + * Please, note that we check without grabbing the slab_mutex. It's safe
    + * because at this moment the children list can't grow.
    + */
    + if (!list_empty(&s->memcg_params.children))
    + rcu_barrier();
    }
    #else
    static inline int shutdown_memcg_caches(struct kmem_cache *s)

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