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

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

On Wed, Aug 28, 2024 at 12:42:02PM GMT, Yosry Ahmed wrote:
> 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.

Do you mean adding a new SLAB_KMALLOC_NORMAL? I will wait for SLAB
maintainers for their opinion on that. BTW this kind of checks are in
the kernel particularly for gfp flags.