Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem_caches for all allocations

[Roman Gushchin]

On Thu, Jun 18, 2020 at 09:33:08AM +0200, Vlastimil Babka wrote:
> On 6/18/20 2:35 AM, Roman Gushchin wrote:
> > On Wed, Jun 17, 2020 at 04:35:28PM -0700, Andrew Morton wrote:
> >> On Mon, 8 Jun 2020 16:06:52 -0700 Roman Gushchin <guro@xxxxxx> wrote:
> >> 
> >> > Instead of having two sets of kmem_caches: one for system-wide and
> >> > non-accounted allocations and the second one shared by all accounted
> >> > allocations, we can use just one.
> >> > 
> >> > The idea is simple: space for obj_cgroup metadata can be allocated
> >> > on demand and filled only for accounted allocations.
> >> > 
> >> > It allows to remove a bunch of code which is required to handle
> >> > kmem_cache clones for accounted allocations. There is no more need
> >> > to create them, accumulate statistics, propagate attributes, etc.
> >> > It's a quite significant simplification.
> >> > 
> >> > Also, because the total number of slab_caches is reduced almost twice
> >> > (not all kmem_caches have a memcg clone), some additional memory
> >> > savings are expected. On my devvm it additionally saves about 3.5%
> >> > of slab memory.
> >> > 
> >> 
> >> This ran afoul of Vlastimil's "mm, slab/slub: move and improve
> >> cache_from_obj()"
> >> (http://lkml.kernel.org/r/20200610163135.17364-10-vbabka@xxxxxxx).  I
> >> resolved things as below.  Not too sure about slab.c's
> >> cache_from_obj()...
> > 
> > It can actually be as simple as:
> > static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> > {
> > 	return s;
> > }
> > 
> > But I wonder if we need it at all, or maybe we wanna rename it to
> > something like obj_check_kmem_cache(void *obj, struct kmem_cache *s),
> > because it has now only debug purposes.
> > 
> > Let me and Vlastimil figure it out and send a follow-up patch.
> > Your version is definitely correct.
> 
> Well, Kees wants to restore the common version of cache_from_obj() [1] for SLAB
> hardening.
> 
> To prevent all that back and forth churn entering git history, I think the best
> is for me to send a -fix to my patch that is functionally same while keeping the
> common function, and then this your patch should only have a minor conflict and
> Kees can rebase his patches on top to become much smaller?

Sounds good to me!

Thanks!