Re: [RFC PATCH V3 0/7] mm/slab: reduce slab accounting memory overhead by allocating slabobj_ext metadata within unused slab space

[Vlastimil Babka <vbabka@suse.cz>]

On 10/27/25 13:28, Harry Yoo wrote:
> RFC v2: https://lore.kernel.org/linux-mm/20250827113726.707801-1-harry.yoo@oracle.com/
> 
> RFC v2 -> v3:
>   - RFC v3 now depends on the patch "[PATCH V2] mm/slab: ensure all metadata
>     in slab object are word-aligned"

Looks like there's some outstanding feedback on that patch. Also on this
series already, so I'll wait for the next version before looking in detail,
but overall it looks good to me! Thanks!

>   - During the merge window, the size of ext4 inode cache has shrunken
>     and it couldn't benefit from the change anymore as the unused space
>     became smaller. But I somehow found a way to shrink the size of
>     ext4 inode object by a word...
> 
>     With new patch 1 and 2, now it can benefit from the optimization again.
> 
>   - As suggested by Andrey, SLUB now disables KASAN and KMSAN, and reset the
>     kasan tag instead of unpoisoning slabobj_ext metadata (Patch 5).
> 
> When CONFIG_MEMCG and CONFIG_MEM_ALLOC_PROFILING are enabled,
> the kernel allocates two pointers per object: one for the memory cgroup
> (obj_cgroup) to which it belongs, and another for the code location
> that requested the allocation.
> 
> In two special cases, this overhead can be eliminated by allocating
> slabobj_ext metadata from unused space within a slab:
> 
>   Case 1. The "leftover" space after the last slab object is larger than
>           the size of an array of slabobj_ext.
> 
>   Case 2. The per-object alignment padding is larger than
>           sizeof(struct slabobj_ext).
> 
> For these two cases, one or two pointers can be saved per slab object.
> Examples: ext4 inode cache (case 1) and xfs inode cache (case 2).
> That's approximately 0.7-0.8% (memcg) or 1.5-1.6%% (memcg + mem profiling)
> of the total inode cache size.
> 
> Implementing case 2 is not straightforward, because the existing code
> assumes that slab->obj_exts is an array of slabobj_ext, while case 2
> breaks the assumption.
> 
> As suggested by Vlastimil, abstract access to individual slabobj_ext
> metadata via a new helper named slab_obj_ext():
> 
> static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
>                                                unsigned long obj_exts,
>                                                unsigned int index)
> {
>         return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index);
> } 
> 
> In the normal case (including case 1), slab->obj_exts points to an array
> of slabobj_ext, and the stride is sizeof(struct slabobj_ext).
> 
> In case 2, the stride is s->size and
> slab->obj_exts = slab_address(slab) + s->red_left_pad + (offset of slabobj_ext)
> 
> With this approach, the memcg charging fastpath doesn't need to care the
> storage method of slabobj_ext.
> 
> Harry Yoo (7):
>   mm/slab: allow specifying freepointer offset when using constructor
>   ext4: specify the free pointer offset for ext4_inode_cache
>   mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper
>   mm/slab: use stride to access slabobj_ext
>   mm/memcontrol,alloc_tag: handle slabobj_ext access under KASAN poison
>   mm/slab: save memory by allocating slabobj_ext array from leftover
>   mm/slab: place slabobj_ext metadata in unused space within s->size
> 
>  fs/ext4/super.c      |  20 ++-
>  include/linux/slab.h |   9 ++
>  mm/memcontrol.c      |  34 +++--
>  mm/slab.h            |  94 ++++++++++++-
>  mm/slab_common.c     |   8 +-
>  mm/slub.c            | 304 ++++++++++++++++++++++++++++++++++++-------
>  6 files changed, 398 insertions(+), 71 deletions(-)
>