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List-Subscribe: List-Unsubscribe: On Mon, Jan 05, 2026 at 05:02:22PM +0900, Harry Yoo wrote: > Happy new year! > > V4: https://lore.kernel.org/linux-mm/20251027122847.320924-1-harry.yoo@oracle.com Actually, that's RFC V3. V4: https://lore.kernel.org/linux-mm/20251222110843.980347-1-harry.yoo@oracle.com/ -- Cheers, Harry / Hyeonggon > 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!) > > 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(-) > > -- > 2.43.0 >