Re: [PATCH v1] memcg: add charging of already allocated slab objects

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Aug 29, 2024 at 10:36:01AM GMT, Muchun Song wrote:
> 
> 
> > On Aug 29, 2024, at 03:03, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > 
> > Hi Muchun,
> > 
> > On Wed, Aug 28, 2024 at 10:36:06AM GMT, Muchun Song wrote:
> >> 
> >> 
> >>> On Aug 28, 2024, at 01:23, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >>> 
> > [...]
> >>>> 
> >>>> Does it handle the case of a too-big-to-be-a-slab-object allocation?
> >>>> I think it's better to handle it properly. Also, why return false here?
> >>>> 
> >>> 
> >>> Yes I will fix the too-big-to-be-a-slab-object allocations. I presume I
> >>> should just follow the kfree() hanlding on !folio_test_slab() i.e. that
> >>> the given object is the large or too-big-to-be-a-slab-object.
> >> 
> >> Hi Shakeel,
> >> 
> >> If we decide to do this, I suppose you will use memcg_kmem_charge_page
> >> to charge big-object. To be consistent, I suggest renaming kmem_cache_charge
> >> to memcg_kmem_charge to handle both slab object and big-object. And I saw
> >> all the functions related to object charging is moved to memcontrol.c (e.g.
> >> __memcg_slab_post_alloc_hook), so maybe we should also do this for
> >> memcg_kmem_charge?
> >> 
> > 
> > If I understand you correctly, you are suggesting to handle the general
> > kmem charging and slab's large kmalloc (size > KMALLOC_MAX_CACHE_SIZE)
> > together with memcg_kmem_charge(). However that is not possible due to
> > slab path updating NR_SLAB_UNRECLAIMABLE_B stats while no updates for
> > this stat in the general kmem charging path (__memcg_kmem_charge_page in
> > page allocation code path).
> > 
> > Also this general kmem charging path is used by many other users like
> > vmalloc, kernel stack and thus we can not just plainly stuck updates to
> > NR_SLAB_UNRECLAIMABLE_B in that path.
> 
> Sorry, maybe I am not clear . To make sure we are on the same page, let
> me clarify my thought. In your v2, I thought if we can rename
> kmem_cache_charge() to memcg_kmem_charge() since kmem_cache_charge()
> already has handled both big-slab-object (size > KMALLOC_MAX_CACHE_SIZE)
> and small-slab-object cases. You know, we have a function of
> memcg_kmem_charge_page() which could be used for charging big-slab-object
> but not small-slab-object. So I thought maybe memcg_kmem_charge() is a
> good name for it to handle both cases. And if we do this, how about moving
> this new function to memcontrol.c since all memcg charging functions are
> moved to memcontrol.c instead of slub.c.
> 

Oh you want the core function to be in memcontrol.c. I don't have any
strong opinion where the code should exist but I do want the interface
to still be kmem_cache_charge() because that is what we are providing to
the users which charging slab objects. Yes some of those might be
big-slab-objects but that is transparent to the users.

Anyways, for now I will go with my current approach but on the followup
will explore and discuss with you on which code should exist in which
file. I hope that is acceptable to you.

thanks,
Shakeel