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List-Unsubscribe: On Tue, Jan 13, 2026 at 10:01:16PM +0900, Harry Yoo wrote: > On Tue, Jan 13, 2026 at 01:50:31PM +0100, Vlastimil Babka wrote: > > On 1/13/26 7:18 AM, Harry Yoo wrote: > > > 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 > > > Signed-off-by: Harry Yoo > > > > Does this look OK to you or was there a reason you didn't do it? :) > > > > diff --git a/mm/slub.c b/mm/slub.c > > index ba15df4ca417..deb69bd9646a 100644 > > --- a/mm/slub.c > > +++ b/mm/slub.c > > @@ -981,8 +981,7 @@ static inline bool obj_exts_in_slab(struct kmem_cache *s, struct slab *slab) > > #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT) > > static bool obj_exts_in_object(struct kmem_cache *s, struct slab *slab) > > { > > - return obj_exts_in_slab(s, slab) && > > - (slab_get_stride(slab) == s->size); > > + return obj_exts_in_slab(s, slab) && (s->flags & SLAB_OBJ_EXT_IN_OBJ); > > There was a reason why I didn't do it :) > > In alloc_slab_obj_exts_early(), when both > obj_exts_fit_within_slab_leftover() and (s->flags & SLAB_OBJ_EXT_IN_OBJ) > returns true, it allocates the metadata from the slab's leftover space. > > I noticed it as I saw a slab error in slab_pad_check() complaining that > the padding area was overwritten, but turned out the problem was > because obj_exts_in_object() returning true when it shouldn't. Perhaps a comment like this? diff --git a/mm/slub.c b/mm/slub.c index ba15df4ca417..c40c3559039e 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -981,6 +981,15 @@ static inline bool obj_exts_in_slab(struct kmem_cache *s, struct slab *slab) #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT) static bool obj_exts_in_object(struct kmem_cache *s, struct slab *slab) { + /* + * When SLAB_OBJ_EXT_IN_OBJ is set, slabobj_ext metadata can be stored + * in one of two ways: + * 1. As an array in the slab's leftover space (after the last object) + * 2. Inline with each object (within s->size) + * + * The actual placement is determined by the stride size rather than + * the SLAB_OBJ_EXT_IN_OBJ flag itself. + */ return obj_exts_in_slab(s, slab) && (slab_get_stride(slab) == s->size); }