Re: [PATCH V5 0/8] mm/slab: reduce slab accounting memory overhead by allocating slabobj_ext metadata within unsed slab space

[Vlastimil Babka]

On 1/5/26 09:02, Harry Yoo wrote:
> Happy new year!
> 
> V4: https://lore.kernel.org/linux-mm/20251027122847.320924-1-harry.yoo@xxxxxxxxxx
> V4 -> V5:
> - Patch 4: Fixed returning false when the return type is unsigned long
> - Patch 7: Fixed incorrect calculation of slabobj_ext offset (Thanks Hao!)

Besides the stuff pointed out the rest seemed ok to me. Can you resend with
those addressed, and rebased on slab.git slab/for-7.0/obj_metadata to avoid
a conflict in patch 8/8 with Hao's comment update patch there? I will add
the series on top there then. Thanks!

> When CONFIG_MEMCG and CONFIG_MEM_ALLOC_PROFILING are enabled,
> the kernel allocates two pointers per object: one for the memory cgroup
> (actually, 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 (8):
>   mm/slab: use unsigned long for orig_size to ensure proper metadata
>     align
>   mm/slab: allow specifying free pointer 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 |  39 +++--
>  mm/memcontrol.c      |  31 +++-
>  mm/slab.h            | 120 ++++++++++++++-
>  mm/slab_common.c     |   8 +-
>  mm/slub.c            | 345 +++++++++++++++++++++++++++++++++++--------
>  6 files changed, 466 insertions(+), 97 deletions(-)
>