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List-Subscribe: List-Unsubscribe: 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" - 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(-) -- 2.43.0