Re: [PATCH 1/6] slab: cleanup kmem_cache_create_memcg()

[Michal Hocko]

On Thu 19-12-13 10:31:43, Vladimir Davydov wrote:
> On 12/18/2013 08:56 PM, Michal Hocko wrote:
> > On Wed 18-12-13 17:16:52, Vladimir Davydov wrote:
> >> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxx>
> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> >> Cc: Glauber Costa <glommer@xxxxxxxxx>
> >> Cc: Christoph Lameter <cl@xxxxxxxxx>
> >> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Dunno, is this really better to be worth the code churn?
> >
> > It even makes the generated code tiny bit bigger:
> > text    data     bss     dec     hex filename
> > 4355     171     236    4762    129a mm/slab_common.o.after
> > 4342     171     236    4749    128d mm/slab_common.o.before
> >
> > Or does it make the further changes much more easier? Be explicit in the
> > patch description if so.
> 
> Hi, Michal
> 
> IMO, undoing under labels looks better than inside conditionals, because
> we don't have to repeat the same deinitialization code then, like this
> (note three calls to kmem_cache_free()):

Agreed but the resulting code is far from doing nice undo on different
conditions. You have out_free_cache which frees everything regardless
whether name or cache registration failed. So it doesn't help with
readability much IMO.

> 
>     s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
>     if (s) {
>         s->object_size = s->size = size;
>         s->align = calculate_alignment(flags, align, size);
>         s->ctor = ctor;
> 
>         if (memcg_register_cache(memcg, s, parent_cache)) {
>             kmem_cache_free(kmem_cache, s);
>             err = -ENOMEM;
>             goto out_locked;
>         }
> 
>         s->name = kstrdup(name, GFP_KERNEL);
>         if (!s->name) {
>             kmem_cache_free(kmem_cache, s);
>             err = -ENOMEM;
>             goto out_locked;
>         }
> 
>         err = __kmem_cache_create(s, flags);
>         if (!err) {
>             s->refcount = 1;
>             list_add(&s->list, &slab_caches);
>             memcg_cache_list_add(memcg, s);
>         } else {
>             kfree(s->name);
>             kmem_cache_free(kmem_cache, s);
>         }
>     } else
>         err = -ENOMEM;
> 
> The next patch, which fixes the memcg_params leakage on error, would
> make it even worse introducing two calls to memcg_free_cache_params()
> after kstrdup and __kmem_cache_create.
> 
> If you think it isn't worthwhile applying this patch, just let me know,
> I don't mind dropping it.

As I've said if it helps with the later patches then I do not mind but
on its own it doesn't sound like a huge improvement.

Btw. you do not have to set err = -ENOMEM before goto out_locked. Just
set before kmem_cache_zalloc. You also do not need to initialize it to 0
because kmem_cache_sanity_check will set it.

> Anyway, I'll improve the comment and resend.

Thanks!

> Thanks.
> 
> >
> >> ---
> >>  mm/slab_common.c |   66 +++++++++++++++++++++++++++---------------------------
> >>  1 file changed, 33 insertions(+), 33 deletions(-)
> >>
> >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> index 0b7bb39..5d6f743 100644
> >> --- a/mm/slab_common.c
> >> +++ b/mm/slab_common.c
> >> @@ -176,8 +176,9 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
> >>  	get_online_cpus();
> >>  	mutex_lock(&slab_mutex);
> >>  
> >> -	if (!kmem_cache_sanity_check(memcg, name, size) == 0)
> >> -		goto out_locked;
> >> +	err = kmem_cache_sanity_check(memcg, name, size);
> >> +	if (err)
> >> +		goto out_unlock;
> >>  
> >>  	/*
> >>  	 * Some allocators will constraint the set of valid flags to a subset
> >> @@ -189,45 +190,41 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
> >>  
> >>  	s = __kmem_cache_alias(memcg, name, size, align, flags, ctor);
> >>  	if (s)
> >> -		goto out_locked;
> >> +		goto out_unlock;
> >>  
> >>  	s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
> >> -	if (s) {
> >> -		s->object_size = s->size = size;
> >> -		s->align = calculate_alignment(flags, align, size);
> >> -		s->ctor = ctor;
> >> -
> >> -		if (memcg_register_cache(memcg, s, parent_cache)) {
> >> -			kmem_cache_free(kmem_cache, s);
> >> -			err = -ENOMEM;
> >> -			goto out_locked;
> >> -		}
> >> +	if (!s) {
> >> +		err = -ENOMEM;
> >> +		goto out_unlock;
> >> +	}
> >>  
> >> -		s->name = kstrdup(name, GFP_KERNEL);
> >> -		if (!s->name) {
> >> -			kmem_cache_free(kmem_cache, s);
> >> -			err = -ENOMEM;
> >> -			goto out_locked;
> >> -		}
> >> +	s->object_size = s->size = size;
> >> +	s->align = calculate_alignment(flags, align, size);
> >> +	s->ctor = ctor;
> >>  
> >> -		err = __kmem_cache_create(s, flags);
> >> -		if (!err) {
> >> -			s->refcount = 1;
> >> -			list_add(&s->list, &slab_caches);
> >> -			memcg_cache_list_add(memcg, s);
> >> -		} else {
> >> -			kfree(s->name);
> >> -			kmem_cache_free(kmem_cache, s);
> >> -		}
> >> -	} else
> >> +	s->name = kstrdup(name, GFP_KERNEL);
> >> +	if (!s->name) {
> >>  		err = -ENOMEM;
> >> +		goto out_free_cache;
> >> +	}
> >> +
> >> +	err = memcg_register_cache(memcg, s, parent_cache);
> >> +	if (err)
> >> +		goto out_free_cache;
> >>  
> >> -out_locked:
> >> +	err = __kmem_cache_create(s, flags);
> >> +	if (err)
> >> +		goto out_free_cache;
> >> +
> >> +	s->refcount = 1;
> >> +	list_add(&s->list, &slab_caches);
> >> +	memcg_cache_list_add(memcg, s);
> >> +
> >> +out_unlock:
> >>  	mutex_unlock(&slab_mutex);
> >>  	put_online_cpus();
> >>  
> >>  	if (err) {
> >> -
> >>  		if (flags & SLAB_PANIC)
> >>  			panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n",
> >>  				name, err);
> >> @@ -236,11 +233,14 @@ out_locked:
> >>  				name, err);
> >>  			dump_stack();
> >>  		}
> >> -
> >>  		return NULL;
> >>  	}
> >> -
> >>  	return s;
> >> +
> >> +out_free_cache:
> >> +	kfree(s->name);
> >> +	kmem_cache_free(kmem_cache, s);
> >> +	goto out_unlock;
> >>  }
> >>  
> >>  struct kmem_cache *
> >> -- 
> >> 1.7.10.4
> >>
> 

-- 
Michal Hocko
SUSE Labs
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