Re: [PATCH 0/2] fix kmem over-charging for embedded obj_exts array

[vbabka]

On 3/11/26 04:06, Hao Li wrote:
> On Wed, Mar 11, 2026 at 11:13:18AM +0900, Harry Yoo wrote:
>> On Tue, Mar 10, 2026 at 11:38:02AM +0000, ranxiaokai627@xxxxxxx wrote:
>> > From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>> > 
>> > Since commit a77d6d338685 ("mm/slab: place slabobj_ext metadata
>> > in unused space within s->size"), the struct slabobj_ext array can
>> > use slab leftover space or be embedded into the slub object to save
>> > memory. In these cases, no extra kmalloc space is allocated for the
>> > obj_exts array.
>> > 
>> > However, obj_full_size() always returns extra sizeof(struct obj_cgroup *)
>> > bytes for every object, which leads to over-charging for slabs with
>> > embedded obj_exts.
>> > 
>> > This series optimizes obj_full_size() to check whether obj_exts uses
>> > slab leftover space or is embedded in the object. If so, only the object
>> > size is charged. Otherwise, the extra obj_cgroup pointer space is also
>> > charged.
>> 
>> Hi Ran,
>> 
>> At first look, I'm not sure if it's a good idea - although it's
>> allocated from wasted space, it's still memory that's needed to
>> charge objects.
> 
> Yes, I've been thinking about this as well.
> 
> For slabobj_ext that lives at the end of the whole slab, it seems reasonable to
> charge it to the cgroup.
> 
>> 
>> But for "embedded into the slub object" case, yeah,
>> the metadata is charged twice, as it's already included in s->size.
> 
> While reading patch 2, I was also thinking about whether it would make sense to
> call obj_exts_in_slab() on every object allocation and free for this case...
> 
> I wonder if we could use objext_flags to carry a bit of information about where
> slabobj_ext is located.

Uh maybe let's not complicate it and just drop obj_full_size() altogether
and just charge s->size? Just accept that allocating slab objects has some
overhead, we're not accounting all of it fully anyway. It shouldn't cause a
big difference in practice.