On Thu, 5 Nov 2015 11:38:43 +0300
Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> wrote:
> On Thu, Nov 05, 2015 at 02:06:21PM +0900, Joonsoo Kim wrote:
> > On Thu, Oct 29, 2015 at 02:05:31PM +0100, Jesper Dangaard Brouer wrote:
> > > Initial implementation missed support for kmem cgroup support
> > > in kmem_cache_free_bulk() call, add this.
> > >
> > > If CONFIG_MEMCG_KMEM is not enabled, the compiler should
> > > be smart enough to not add any asm code.
> > >
> > > Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> > > ---
> > > mm/slub.c | 3 +++
> > > 1 file changed, 3 insertions(+)
> > >
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 9be12ffae9fc..9875864ad7b8 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -2845,6 +2845,9 @@ static int build_detached_freelist(struct kmem_cache *s, size_t size,
> > > if (!object)
> > > return 0;
> > >
> > > + /* Support for kmemcg */
> > > + s = cache_from_obj(s, object);
> > > +
> > > /* Start new detached freelist */
> > > set_freepointer(s, object, NULL);
> > > df->page = virt_to_head_page(object);
> >
> > Hello,
> >
> > It'd better to add this 's = cache_from_obj()' on kmem_cache_free_bulk().
> > Not only build_detached_freelist() but also slab_free() need proper
> > cache.
>
> Yeah, Joonsoo is right.
But cache_from_obj() takes an object as input and in kmem_cache_free_bulk()
that object is not directly available... Could send "s" as a reference
(to build_detached_freelist) to allow re-assignment of "s" so
slab_free() gets the correct "s". But it will not look pretty...
Else we can get the object via: p[size -1] which also look a little
funny... but it might not be correct in-case NULL pointers in the input
p-array.
> Besides, there's a bug in kmem_cache_alloc_bulk:
Thanks for spotting this!!!
> > /* Note that interrupts must be enabled when calling this function. */
> > bool kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> > void **p)
> > {
> > struct kmem_cache_cpu *c;
> > int i;
> >
> > /*
> > * Drain objects in the per cpu slab, while disabling local
> > * IRQs, which protects against PREEMPT and interrupts
> > * handlers invoking normal fastpath.
> > */
> > local_irq_disable();
> > c = this_cpu_ptr(s->cpu_slab);
> >
> > for (i = 0; i < size; i++) {
> > void *object = c->freelist;
> >
> > if (unlikely(!object)) {
> > /*
> > * Invoking slow path likely have side-effect
> > * of re-populating per CPU c->freelist
> > */
> > p[i] = ___slab_alloc(s, flags, NUMA_NO_NODE,
> > _RET_IP_, c);
> > if (unlikely(!p[i]))
> > goto error;
> >
> > c = this_cpu_ptr(s->cpu_slab);
> > continue; /* goto for-loop */
> > }
> >
> > /* kmem_cache debug support */
> > s = slab_pre_alloc_hook(s, flags);
>
> slab_pre_alloc_hook expects a global cache and returns per memcg one, so
> calling this function from inside a kmemcg will result in hitting the
> VM_BUG_ON in __memcg_kmem_get_cache, not saying about mis-accounting of
> __slab_alloc.
>
> memcg_kmem_get_cache should be called once, in the very beginning of
> kmem_cache_alloc_bulk, and it should be matched by memcg_kmem_put_cache
> when we are done.
To solve this correctly it looks like I need to pull out
memcg_kmem_put_cache(s) call in the slab_post_alloc_hook() call.
>
> > if (unlikely(!s))
> > goto error;
> >
> > c->freelist = get_freepointer(s, object);
> > p[i] = object;
> >
> > /* kmem_cache debug support */
> > slab_post_alloc_hook(s, flags, object);
> > }
> > c->tid = next_tid(c->tid);
> > local_irq_enable();
> >
> > /* Clear memory outside IRQ disabled fastpath loop */
> > if (unlikely(flags & __GFP_ZERO)) {
> > int j;
> >
> > for (j = 0; j < i; j++)
> > memset(p[j], 0, s->object_size);
> > }
> >
> > return true;
> >
> > error:
> > __kmem_cache_free_bulk(s, i, p);
> > local_irq_enable();
> > return false;
> > }
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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