Re: [PATCH v2] memcg: add charging of already allocated slab objects

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Aug 28, 2024 at 12:14 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Tue, Aug 27, 2024 at 05:34:24PM GMT, Yosry Ahmed wrote:
> > On Tue, Aug 27, 2024 at 4:52 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> [...]
> > > +
> > > +#define KMALLOC_TYPE (SLAB_KMALLOC | SLAB_CACHE_DMA | \
> > > +                     SLAB_ACCOUNT | SLAB_RECLAIM_ACCOUNT)
> > > +
> > > +static __fastpath_inline
> > > +bool memcg_slab_post_charge(void *p, gfp_t flags)
> > > +{
> > > +       struct slabobj_ext *slab_exts;
> > > +       struct kmem_cache *s;
> > > +       struct folio *folio;
> > > +       struct slab *slab;
> > > +       unsigned long off;
> > > +
> > > +       folio = virt_to_folio(p);
> > > +       if (!folio_test_slab(folio)) {
> > > +               return __memcg_kmem_charge_page(folio_page(folio, 0), flags,
> > > +                                               folio_order(folio)) == 0;
> >
> > Will this charge the folio again if it was already charged? It seems
> > like we avoid this for already charged slab objects below but not
> > here.
> >
>
> Thanks for catchig this. It's an easy fix and will do in v3.
>
> > > +       }
> > > +
> > > +       slab = folio_slab(folio);
> > > +       s = slab->slab_cache;
> > > +
> > > +       /* Ignore KMALLOC_NORMAL cache to avoid circular dependency. */
> > > +       if ((s->flags & KMALLOC_TYPE) == SLAB_KMALLOC)
> > > +               return true;
> >
> > Would it be clearer to check if the slab cache is one of
> > kmalloc_caches[KMALLOC_NORMAL]? This should be doable by comparing the
> > address of the slab cache with the addresses of
> > kmalloc_cache[KMALLOC_NORMAL] (perhaps in a helper). I need to refer
> > to your reply to Roman to understand why this works.
> >
>
> Do you mean looping over kmalloc_caches[KMALLOC_NORMAL] and comparing
> the given slab cache address? Nah man why do long loop of pointer
> comparisons when we can simply check the flag of the given kmem cache.
> Also this array will increase with the recent proposed random kmalloc
> caches.

Oh I thought kmalloc_caches[KMALLOC_NORMAL] is an array of the actual
struct kmem_cache objects, so I thought we can just check if:
s >= kmalloc_caches[KMALLOC_NORMAL][0] &&
s >= kmalloc_caches[KMALLOC_NORMAL][LAST_INDEX]

I just realized it's an array of pointers, so we would need to loop
and compare them.

I still find the flags comparisons unclear and not very future-proof
tbh. I think we can just store the type in struct kmem_cache? I think
there are multiple holes there.