Re: [PATCH RFC 1/4] mm, slab: move memcg charging to post-alloc hook

[Roman Gushchin <roman.gushchin@linux.dev>]

On Tue, Mar 12, 2024 at 06:59:37PM +0000, Matthew Wilcox wrote:
> On Tue, Mar 12, 2024 at 11:52:46AM -0700, Roman Gushchin wrote:
> > On Fri, Mar 01, 2024 at 06:07:08PM +0100, Vlastimil Babka wrote:
> > > @@ -1926,71 +1939,51 @@ static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s,
> > >  			return false;
> > >  	}
> > >  
> > > -	if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s)))
> > > +	if (obj_cgroup_charge(objcg, flags, size * obj_full_size(s)))
> > >  		return false;
> > >  
> > > -	*objcgp = objcg;
> > > +	for (i = 0; i < size; i++) {
> > > +		slab = virt_to_slab(p[i]);
> > 
> > Not specific to this change, but I wonder if it makes sense to introduce virt_to_slab()
> > variant without any extra checks for this and similar cases, where we know for sure
> > that p resides on a slab page. What do you think?
> 
> You'd only save a single test_bit() ... is it really worth doing?
> Cache misses are the expensive thing, not instructions.

I agree here, unlikely it will produce a significant difference.

> And debugging
> time: if somehow p[i] becomes not-on-a-slab-anymore, getting a NULL
> pointer splat here before we go any further might be worth all the CPU
> time wasted doing that test_bit().

Well, Idk if it's a feasible concern here, hard to imagine how p[i]
wouldn't belong to a slab page without something like a major memory
corruption.

Overall I agree it's not a big deal and the current code is fine.

Thanks!