Re: [PATCH RFC v2 3/8] mm, slab: rework KMALLOC_RECLAIM handling of SLUB_TINY

[Hao Li <hao.li@linux.dev>]

On Sat, Oct 24, 2026 at 09:07:25PM +0200, Vlastimil Babka (SUSE) wrote:
> One (relatively minor) aspect of CONFIG_SLUB_TINY is the assumption that
> letting reclaimable and non-reclaimable caches merge will save more
> memory, which is more important on small systems than the page allocator
> level fragmentation (where distinguishing reclaimable slabs should
> help).
> 
> For now that has been achieved by #ifdefs that result in making the
> SLAB_RECLAIM_ACCOUNT no-op and aliasing KMALLOC_RECLAIM to
> KMALLOC_NORMAL on the enum values level.
> 
> With the goal of converting CONFIG_SLUB_TINY to a boot-time decision,
> rework this so that SLAB_RECLAIM_ACCOUNT and KMALLOC_RECLAIM are
> distinctly defined always. Now, creation of kmalloc-rcl caches is
> avoided by assigning a pointer to the normal kmalloc cache on the
> kmalloc_caches array level.
> 
> To prepare disconnecting of this boot-time decision (and the evaluation
> of the need for kmalloc_no_objext) from CONFIG_SLUB_TINY, introduce a
> slub_tiny_enabled variable that's initialized to the CONFIG_SLUB_TINY
> state.
> 
> There is one minor functional change for CONFIG_SLUB_TINY=y. Since
> SLAB_RECLAIM_ACCOUNT is not 0 anymore, caches that differ only in that
> flag will stop being mergeable. If that turns out to have a noticeable
> impact on memory footprint, we can adjust the merging decisions to
> ignore SLAB_RECLAIM_ACCOUNT when slub_tiny_enabled is true.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---
>  include/linux/slab.h | 10 ----------
>  mm/slab.h            |  5 +++--
>  mm/slab_common.c     | 12 ++++++------
>  mm/slub.c            |  2 ++

[...]

> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 02dfe4351e8b..33e76a0c3084 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -835,11 +835,7 @@ EXPORT_SYMBOL(kmalloc_size_roundup);
>  #define KMALLOC_CGROUP_NAME(sz)
>  #endif
>  
> -#ifndef CONFIG_SLUB_TINY
>  #define KMALLOC_RCL_NAME(sz)	.name[KMALLOC_RECLAIM] = "kmalloc-rcl-" #sz,
> -#else
> -#define KMALLOC_RCL_NAME(sz)
> -#endif
>  
>  #ifdef CONFIG_KMALLOC_PARTITION_CACHES
>  #define __KMALLOC_PARTITION_CONCAT(a, b) a ## b
> @@ -976,7 +972,11 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
>  	unsigned int aligned_size = kmalloc_info[idx].size;
>  	int aligned_idx = idx;
>  
> -	if ((KMALLOC_RECLAIM != KMALLOC_NORMAL) && (type == KMALLOC_RECLAIM)) {
> +	if (type == KMALLOC_RECLAIM) {
> +		if (slab_tiny_enabled) {
> +			kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
> +			return;
> +		}
>  		flags |= SLAB_RECLAIM_ACCOUNT;
>  	} else if (IS_ENABLED(CONFIG_MEMCG) && (type == KMALLOC_CGROUP)) {
>  		if (mem_cgroup_kmem_disabled()) {
> @@ -1009,7 +1009,7 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
>  	 * KMALLOC_NO_OBJ_EXT cache.
>  	 */
>  	if (!mem_cgroup_kmem_disabled()) {
> -		if (type == KMALLOC_NORMAL && KMALLOC_RECLAIM != KMALLOC_NORMAL)
> +		if (type == KMALLOC_NORMAL && !slab_tiny_enabled)

(While reviewing Harry's patch [1], I happened to notice a small inconsistency
in the current code. Since these lines are being touched in this series, I
figured I'd bring it up here, even though I haven't properly started reviewing
this series yet :D)

IIUC, for both KMALLOC_NORMAL and KMALLOC_PARTITION_START + n, if gfp has
__GFP_ACCOUNT set (and without __GFP_DMA/__GFP_RECLAIMABLE), the allocation is
redirected to KMALLOC_CGROUP. This seems to mean that neither KMALLOC_NORMAL
nor KMALLOC_PARTITION_START + n actually needs an objcg slot in obj_exts.

Right now, KMALLOC_NORMAL already doesn't carry SLAB_MAY_ACCOUNT, but
KMALLOC_PARTITION_START + n still does. Given that, would it make sense to
change this line to something like:

if (type >= KMALLOC_PARTITION_START && type <= KMALLOC_PARTITION_END &&
	!slab_tiny_enabled)

[1] https://lore.kernel.org/linux-mm/20261021-memalloc-prof-dont-waste-slabobj-ext-metadata-v1-0-096800aa3330@kernel.org/

>  			flags |= SLAB_NO_MERGE;
>  		else if (!(flags & SLAB_NO_OBJ_EXT))
>  			flags |= SLAB_MAY_ACCOUNT;
> diff --git a/mm/slub.c b/mm/slub.c
> index 51daf361c601..a3ee1bdf0eb3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -219,6 +219,8 @@ DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>  			slab_obj_ext_has_codetag_key);
>  #endif
>  
> +bool slab_tiny_enabled __read_mostly = IS_ENABLED(CONFIG_SLUB_TINY);
> +
>  /* Structure holding extra parameters for slab allocations */
>  struct slab_alloc_context {
>  	unsigned long caller_addr;
> 
> -- 
> 2.56.0
>