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SubjectRe: [PATCH] mm: fix race between kmem_cache destroy, create and deactivate
On Mon, May 21, 2018 at 11:42 AM Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Mon, 21 May 2018 10:41:16 -0700 Shakeel Butt <shakeelb@google.com>
wrote:

> > The memcg kmem cache creation and deactivation (SLUB only) is
> > asynchronous. If a root kmem cache is destroyed whose memcg cache is in
> > the process of creation or deactivation, the kernel may crash.
> >
> > Example of one such crash:
> > general protection fault: 0000 [#1] SMP PTI
> > CPU: 1 PID: 1721 Comm: kworker/14:1 Not tainted 4.17.0-smp
> > ...
> > Workqueue: memcg_kmem_cache kmemcg_deactivate_workfn
> > RIP: 0010:has_cpu_slab
> > ...
> > Call Trace:
> > ? on_each_cpu_cond
> > __kmem_cache_shrink
> > kmemcg_cache_deact_after_rcu
> > kmemcg_deactivate_workfn
> > process_one_work
> > worker_thread
> > kthread
> > ret_from_fork+0x35/0x40
> >
> > This issue is due to the lack of reference counting for the root
> > kmem_caches. There exist a refcount in kmem_cache but it is actually a
> > count of aliases i.e. number of kmem_caches merged together.
> >
> > This patch make alias count explicit and adds reference counting to the
> > root kmem_caches. The reference of a root kmem cache is elevated on
> > merge and while its memcg kmem_cache is in the process of creation or
> > deactivation.
> >

> The patch seems depressingly complex.

> And a bit underdocumented...


I will add more documentation to the code.

> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -674,6 +674,8 @@ struct memcg_cache_params {
> > };
> >
> > int memcg_update_all_caches(int num_memcgs);
> > +bool kmem_cache_tryget(struct kmem_cache *s);
> > +void kmem_cache_put(struct kmem_cache *s);
> >
> > /**
> > * kmalloc_array - allocate memory for an array.
> > diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
> > index d9228e4d0320..4bb22c89a740 100644
> > --- a/include/linux/slab_def.h
> > +++ b/include/linux/slab_def.h
> > @@ -41,7 +41,8 @@ struct kmem_cache {
> > /* 4) cache creation/removal */
> > const char *name;
> > struct list_head list;
> > - int refcount;
> > + refcount_t refcount;
> > + int alias_count;

> The semantic meaning of these two? What locking protects alias_count?

SLAB and SLUB allow reusing existing root kmem caches. The alias_count of a
kmem cache tells the number of times this kmem cache is reused (maybe
shared_count or reused_count are better names). Basically if there were 5
root kmem cache creation request and suppose SLAB/SLUB decide to reuse the
first kmem cache created for next 4 requests then this count will be 5 and
all 5 will be pointing to the same kmem_cache object.

Before this patch, alias_count (previously named refcount) was modified
only within slab_mutex but can be read outside. It was conflated into
multiple things like shared count, reference count and unmergeable flag (if
-ve). This patch decouples the reference counting from this field and there
is no need to protect alias_count with locks.

> > int object_size;
> > int align;
> >
> > diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> > index 3773e26c08c1..532d4b6f83ed 100644
> > --- a/include/linux/slub_def.h
> > +++ b/include/linux/slub_def.h
> > @@ -97,7 +97,8 @@ struct kmem_cache {
> > struct kmem_cache_order_objects max;
> > struct kmem_cache_order_objects min;
> > gfp_t allocflags; /* gfp flags to use on each alloc */
> > - int refcount; /* Refcount for slab cache destroy */
> > + refcount_t refcount; /* Refcount for slab cache destroy */
> > + int alias_count; /* Number of root kmem caches merged */

> "merged" what with what in what manner?

shared or reused might be better words here.

> > void (*ctor)(void *);
> > unsigned int inuse; /* Offset to metadata */
> > unsigned int align; /* Alignment */
> >
> > ...
> >
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -25,7 +25,8 @@ struct kmem_cache {
> > unsigned int useroffset;/* Usercopy region offset */
> > unsigned int usersize; /* Usercopy region size */
> > const char *name; /* Slab name for sysfs */
> > - int refcount; /* Use counter */
> > + refcount_t refcount; /* Use counter */
> > + int alias_count;

> Semantic meaning/usage of alias_count? Locking for it?

Will add in the next version.

> > void (*ctor)(void *); /* Called on object slot creation */
> > struct list_head list; /* List of all slab caches on the system
*/
> > };
> >
> > ...
> >
> > +bool kmem_cache_tryget(struct kmem_cache *s)
> > +{
> > + if (is_root_cache(s))
> > + return refcount_inc_not_zero(&s->refcount);
> > + return false;
> > +}
> > +
> > +void kmem_cache_put(struct kmem_cache *s)
> > +{
> > + if (is_root_cache(s) &&
> > + refcount_dec_and_test(&s->refcount))
> > + __kmem_cache_destroy(s, true);
> > +}
> > +
> > +void kmem_cache_put_locked(struct kmem_cache *s)
> > +{
> > + if (is_root_cache(s) &&
> > + refcount_dec_and_test(&s->refcount))
> > + __kmem_cache_destroy(s, false);
> > +}

> Some covering documentation for the above would be useful. Why do they
> exist, why do they only operate on the root cache? etc.

Ack.

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