mm/slab: place slabobj_ext metadata in unused space within s->size

When a cache has high s->align value and s->object_size is not aligned
to it, each object ends up with some unused space because of alignment.
If this wasted space is big enough, we can use it to store the
slabobj_ext metadata instead of wasting it.

On my system, this happens with caches like kmem_cache, mm_struct, pid,
task_struct, sighand_cache, xfs_inode, and others.

To place the slabobj_ext metadata within each object, the existing
slab_obj_ext() logic can still be used by setting:

  - slab->obj_exts = slab_address(slab) + (slabobj_ext offset)
  - stride = s->size

slab_obj_ext() doesn't need know where the metadata is stored,
so this method works without adding extra overhead to slab_obj_ext().

A good example benefiting from this optimization is xfs_inode
(object_size: 992, align: 64). To measure memory savings, 2 millions of
files were created on XFS.

[ MEMCG=y, MEM_ALLOC_PROFILING=n ]

Before patch (creating ~2.64M directories on xfs):
  Slab:            5175976 kB
  SReclaimable:    3837524 kB
  SUnreclaim:      1338452 kB

After patch (creating ~2.64M directories on xfs):
  Slab:            5152912 kB
  SReclaimable:    3838568 kB
  SUnreclaim:      1314344 kB (-23.54 MiB)

Enjoy the memory savings!

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260113061845.159790-10-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

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