Re: [PATCH] mm/slub: serve slabobj_ext array from a strictly larger kmalloc cache

[Harry Yoo <harry@kernel.org>]

On 6/30/26 3:12 PM, Vlastimil Babka (SUSE) wrote:
> On 6/30/26 07:29, Suren Baghdasaryan wrote:
>> On Mon, Jun 29, 2026 at 9:42 PM Harry Yoo <harry@kernel.org> wrote:
>>>
>>>
>>>
>>> On 6/30/26 1:39 PM, Suren Baghdasaryan wrote:
>>>> On Mon, Jun 29, 2026 at 9:38 PM Suren Baghdasaryan <surenb@google.com> wrote:
>>>>>
>>>
>>> Ah, here I meant backporting either the kmalloc_flags()+KMALLOC_TYPE or
>>> SLAB_BUCKETS approach.
>>>
>>>>> Yes, it's worth backporting, so we can merge Shakeel's change as is
>>>
>>> Right.
>>>
>>>>> and then once Vlastimil's patch is merged we can implement the new
>>>
>>> Vlastimil's patch has already landed mainline, by the way :)
>>
>> Nice! I suggest posting Shakeel's patch CC'ing stable for backports
>> and then following up with the fix using KMALLOC_TYPE. Vlastimil,
>> WDYT?
> 
> Sounded like a plan, but then I realized I misunderstood the amount of the
> wastage. E.g. on my system kmalloc-8k with 4 objects per slab would have
> obj_ext size of 64, but now it's 16k? That's ridiculous.

Right.

...which is why I was assuming either the KMALLOC_TYPE or SLAB_BUCKETS
approach would be backported as a follow-up. Err, should have
communicated clearly, apologies.

> I think it will> even self-amplify to some extent? kmalloc-8 would
have 512 objects per slab,
> so its obj_ext is 8k. It will not recursively create an obj_ext for the> obj_ext, but other 8k allocation in the same kmalloc-8k slab could then
> trigger it, right?

True, assuming that by 'self-amplifying' you meant this patch creates
more kmalloc-8k objects, and also now kmalloc-8k wastes memory memory.

> We could say it's for a debugging feature, but also it's running in
> production fleets (and Android?), so probably not that easy to dismiss.

I think a key factor is when it's enabled in production.

kconfigs says Android selects MEM_ALLOC_PROFILING, but not
MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT.

I assumed that turning it on by default in the entire fleet
would be bit hard to justify... (please correct me,
if it's not the case)

> Sudden memory increase in a LTS due to this backport doesn't sound nice to me.

-- 
Cheers,
Harry / Hyeonggon