Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

[Roman Gushchin <guro@fb.com>]

On Wed, May 05, 2021 at 02:31:28PM -0400, Waiman Long wrote:
> On 5/5/21 2:02 PM, Vlastimil Babka wrote:
> > On 5/5/21 7:30 PM, Roman Gushchin wrote:
> > > On Wed, May 05, 2021 at 11:46:13AM -0400, Waiman Long wrote:
> > > > With this change, all the objcg pointer array objects will come from
> > > > KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So
> > > > both the recursive kfree() problem and non-freeable slab problem are
> > > > gone. Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer
> > > > have mixed accounted and unaccounted objects, this will slightly reduce
> > > > the number of objcg pointer arrays that need to be allocated and save
> > > > a bit of memory.
> > > Unfortunately the positive effect of this change will be likely
> > > reversed by a lower utilization due to a larger number of caches.
> > > 
> > > Btw, I wonder if we also need a change in the slab caches merging procedure?
> > > KMALLOC_NORMAL caches should not be merged with caches which can potentially
> > > include accounted objects.
> > Good point. But looks like kmalloc* caches are extempt from all merging in
> > create_boot_cache() via
> > 
> > 	s->refcount = -1;       /* Exempt from merging for now */
> > 
> > It wouldn't hurt though to create the kmalloc-cg-* caches with SLAB_ACCOUNT flag
> > to prevent accidental merging in case the above is ever removed. It would also
> > better reflect reality, and ensure that the array is allocated immediately with
> > the page, AFAICS.
> > 
> I am not sure if this is really true.
> 
> struct kmem_cache *__init create_kmalloc_cache(const char *name,
>                 unsigned int size, slab_flags_t flags,
>                 unsigned int useroffset, unsigned int usersize)
> {
>         struct kmem_cache *s = kmem_cache_zalloc(kmem_cache, GFP_NOWAIT);
> 
>         if (!s)
>                 panic("Out of memory when creating slab %s\n", name);
> 
>         create_boot_cache(s, name, size, flags, useroffset, usersize);
>         kasan_cache_create_kmalloc(s);
>         list_add(&s->list, &slab_caches);
>         s->refcount = 1;
>         return s;
> }
> 
> Even though refcount is set to -1 initially, it is set back to 1 afterward.
> So merging can still happen AFAICS.

Right, thanks, I already noticed it. Then yeah, we should make sure we're not
merging KMALLOC_NORMAL caches with any others.