Re: [RFC PATCH V3 6/7] mm/slab: save memory by allocating slabobj_ext array from leftover

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 29, 2025 at 1:00 AM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> On Tue, Oct 28, 2025 at 08:07:42PM -0700, Suren Baghdasaryan wrote:
> > On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@oracle.com> wrote:
> > >
> > > The leftover space in a slab is always smaller than s->size, and
> > > kmem caches for large objects that are not power-of-two sizes tend to have
> > > a greater amount of leftover space per slab. In some cases, the leftover
> > > space is larger than the size of the slabobj_ext array for the slab.
> > >
> > > An excellent example of such a cache is ext4_inode_cache. On my system,
> > > the object size is 1144, with a preferred order of 3, 28 objects per slab,
> > > and 736 bytes of leftover space per slab.
> > >
> > > Since the size of the slabobj_ext array is only 224 bytes (w/o mem
> > > profiling) or 448 bytes (w/ mem profiling) per slab, the entire array
> > > fits within the leftover space.
> > >
> > > Allocate the slabobj_exts array from this unused space instead of using
> > > kcalloc(), when it is large enough. The array is always allocated when
> > > creating new slabs, because implementing lazy allocation correctly is
> > > difficult without expensive synchronization.
> > >
> > > To avoid unnecessary overhead when MEMCG (with SLAB_ACCOUNT) and
> > > MEM_ALLOC_PROFILING are not used for the cache, only allocate the
> > > slabobj_ext array only when either of them are enabled when slabs are
> > > created.
> > >
> > > [ MEMCG=y, MEM_ALLOC_PROFILING=n ]
> > >
> > > Before patch (creating 2M directories on ext4):
> > >   Slab:            3575348 kB
> > >   SReclaimable:    3137804 kB
> > >   SUnreclaim:       437544 kB
> > >
> > > After patch (creating 2M directories on ext4):
> > >   Slab:            3558236 kB
> > >   SReclaimable:    3139268 kB
> > >   SUnreclaim:       418968 kB (-18.14 MiB)
> > >
> > > Enjoy the memory savings!
> > >
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > > ---
> > >  mm/slub.c | 147 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 142 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 13acc9437ef5..8101df5fdccf 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > +static inline bool obj_exts_in_slab(struct kmem_cache *s, struct slab *slab)
> > > +{
> > > +       unsigned long obj_exts;
> > > +
> > > +       if (!obj_exts_fit_within_slab_leftover(s, slab))
> > > +               return false;
> > > +
> > > +       obj_exts = (unsigned long)slab_address(slab);
> > > +       obj_exts += obj_exts_offset_in_slab(s, slab);
> > > +       return obj_exts == slab_obj_exts(slab);
> >
> > You can check that slab_obj_exts(slab) is not NULL before making the
> > above calculations.
>
> Did you mean this?
>
>   if (!slab_obj_exts(slab))
>       return false;

Yes but you can store the returned value to reuse later in the last
"return obj_exts == slab_obj_exts(slab);" expression.


>
> If so, yes that makes sense.
>
> > > @@ -2185,6 +2311,11 @@ static inline void free_slab_obj_exts(struct slab *slab)
> > >  {
> > >  }
> > >
> > > +static inline void alloc_slab_obj_exts_early(struct kmem_cache *s,
> > > +                                                      struct slab *slab)
> > > +{
> > > +}
> > > +
> > >  #endif /* CONFIG_SLAB_OBJ_EXT */
> > >
> > >  #ifdef CONFIG_MEM_ALLOC_PROFILING
> > > @@ -3155,7 +3286,9 @@ static inline bool shuffle_freelist(struct kmem_cache *s, struct slab *slab)
> > >  static __always_inline void account_slab(struct slab *slab, int order,
> > >                                          struct kmem_cache *s, gfp_t gfp)
> > >  {
> > > -       if (memcg_kmem_online() && (s->flags & SLAB_ACCOUNT))
> > > +       if (memcg_kmem_online() &&
> > > +                       (s->flags & SLAB_ACCOUNT) &&
> > > +                       !slab_obj_exts(slab))
> > >                 alloc_slab_obj_exts(slab, s, gfp, true);
> >
> > Don't you need to add a check for !obj_exts_in_slab() inside
> > alloc_slab_obj_exts() to avoid allocating slab->obj_exts?
>
> slab_obj_exts() should have returned a nonzero value
> and then we don't call alloc_slab_obj_exts()?

Sorry, I mean that you would need to check
obj_exts_fit_within_slab_leftover() inside alloc_slab_obj_exts() to
avoid allocating the vector when obj_exts can fit inside the slab
itself. This is because alloc_slab_obj_exts() can be called from other
places as well. However, from your next comment, I realize that your
intention might have been to keep those other callers intact and
allocate the vector separately even if the obj_exts could have been
squeezed inside the slab. Is that correct?

>
> > >         mod_node_page_state(slab_pgdat(slab), cache_vmstat_idx(s),
> > > @@ -3219,9 +3352,6 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> > >         slab->objects = oo_objects(oo);slab_obj_exts
> > >         slab->inuse = 0;
> > >         slab->frozen = 0;
> > > -       init_slab_obj_exts(slab);
> > > -
> > > -       account_slab(slab, oo_order(oo), s, flags);
> > >
> > >         slab->slab_cache = s;
> > >
> > > @@ -3230,6 +3360,13 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
> > >         start = slab_address(slab);
> > >
> > >         setup_slab_debug(s, slab, start);
> > > +       init_slab_obj_exts(slab);
> > > +       /*
> > > +        * Poison the slab before initializing the slabobj_ext array
> > > +        * to prevent the array from being overwritten.
> > > +        */
> > > +       alloc_slab_obj_exts_early(s, slab);
> > > +       account_slab(slab, oo_order(oo), s, flags);
> >
> >  alloc_slab_obj_exts() is called in 2 other places:
> > 1. __memcg_slab_post_alloc_hook()
> > 2. prepare_slab_obj_exts_hook()
> >
> > Don't you need alloc_slab_obj_exts_early() there as well?
>
> That's good point, and I thought it's difficult to address
> concurrency problem without using a per-slab lock.
>
> Thread A                    Thread B
> - sees slab->obj_exts == 0
>                             - sees slab->obj_exts == 0
>                             - allocates the vector from unused space
>                               and initializes it.
>                             - try cmpxchg()
> - allocates the vector
>   from unused space and
>   initializes it.
>   (the vector is already
>    in use and it's overwritten!)
>
> - try cmpxchg()
>
> But since this is slowpath, using slab_{lock,unlock}() here is probably
> fine. What do you think?

Ok, was your original intent to leave these callers as is and allocate
the vector like we do today even if obj_exts fit inside the slab?

>
> --
> Cheers,
> Harry / Hyeonggon