Re: [PATCH v4 10/22] slab: add optimized sheaf refill from partial list

Harry Yoo
2026-01-29T08:01:47Z

On Fri, Jan 23, 2026 at 07:52:48AM +0100, Vlastimil Babka wrote:
> At this point we have sheaves enabled for all caches, but their refill
> is done via __kmem_cache_alloc_bulk() which relies on cpu (partial)
> slabs - now a redundant caching layer that we are about to remove.
>
> The refill will thus be done from slabs on the node partial list.
> Introduce new functions that can do that in an optimized way as it's
> easier than modifying the __kmem_cache_alloc_bulk() call chain.
>
> Introduce struct partial_bulk_context, a variant of struct
> partial_context that can return a list of slabs from the partial list
> with the sum of free objects in them within the requested min and max.
>
> Introduce get_partial_node_bulk() that removes the slabs from freelist
> and returns them in the list. There is a racy read of slab->counters
> so make sure the non-atomic write in __update_freelist_slow() is not
> tearing.
>
> Introduce get_freelist_nofreeze() which grabs the freelist without
> freezing the slab.
>
> Introduce alloc_from_new_slab() which can allocate multiple objects from
> a newly allocated slab where we don't need to synchronize with freeing.
> In some aspects it's similar to alloc_single_from_new_slab() but assumes
> the cache is a non-debug one so it can avoid some actions. It supports
> the allow_spin parameter, which we always set true here, but the
> followup change will reuse the function in a context where it may be
> false.
>
> Introduce __refill_objects() that uses the functions above to fill an
> array of objects. It has to handle the possibility that the slabs will
> contain more objects that were requested, due to concurrent freeing of
> objects to those slabs. When no more slabs on partial lists are
> available, it will allocate new slabs. It is intended to be only used
> in context where spinning is allowed, so add a WARN_ON_ONCE check there.
>
> Finally, switch refill_sheaf() to use __refill_objects(). Sheaves are
> only refilled from contexts that allow spinning, or even blocking.
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/slub.c | 293 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 272 insertions(+), 21 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 22acc249f9c0..142a1099bbc1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -778,7 +786,8 @@ __update_freelist_slow(struct slab *slab, struct freelist_counters *old,
> slab_lock(slab);
> if (slab->freelist == old->freelist &&
> slab->counters == old->counters) {
> - slab->freelist = new->freelist;
> + /* prevent tearing for the read in get_partial_node_bulk() */
> + WRITE_ONCE(slab->freelist, new->freelist);
> slab->counters = new->counters;
> ret = true;
> }

Other than the above being WRITE_ONCE(slab->counters, new->counters),
Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

--
Cheers,
Harry / Hyeonggon