Re: [RFC PATCH 01/15] static kmem_cache instances for core caches

[Harry Yoo <harry.yoo@oracle.com>]

On Sat, Jan 10, 2026 at 04:02:03AM +0000, Al Viro wrote:
>         kmem_cache_create() and friends create new instances of
> struct kmem_cache and return pointers to those.  Quite a few things in
> core kernel are allocated from such caches; each allocation involves
> dereferencing an assign-once pointer and for sufficiently hot ones that
> dereferencing does show in profiles.
> 
>         There had been patches floating around switching some of those
> to runtime_const infrastructure.  Unfortunately, it's arch-specific
> and most of the architectures lack it.
> 
>         There's an alternative approach applicable at least to the caches
> that are never destroyed, which covers a lot of them.  No matter what,
> runtime_const for pointers is not going to be faster than plain &,
> so if we had struct kmem_cache instances with static storage duration, we
> would be at least no worse off than we are with runtime_const variants.
> 
>         There are obstacles to doing that, but they turn out to be easy
> to deal with.
> 
> 1) as it is, struct kmem_cache is opaque for anything outside of a few
> files in mm/*; that avoids serious headache with header dependencies,
> etc., and it's not something we want to lose.  Solution: struct
> kmem_cache_opaque, with the size and alignment identical to struct
> kmem_cache.  Calculation of size and alignment can be done via the same
> mechanism we use for asm-offsets.h and rq-offsets.h, with build-time
> check for mismatches.  With that done, we get an opaque type defined in
> linux/slab-static.h that can be used for declaring those caches.
> In linux/slab.h we add a forward declaration of kmem_cache_opaque +
> helper (to_kmem_cache()) converting a pointer to kmem_cache_opaque
> into pointer to kmem_cache.
> 
> 2) real constructor of kmem_cache needs to be taught to deal with
> preallocated instances.  That turns out to be easy - we already pass an
> obscene amount of optional arguments via struct kmem_cache_args, so we
> can stash the pointer to preallocated instance in there.  Changes in
> mm/slab_common.c are very minor - we should treat preallocated caches
> as unmergable, use the instance passed to us instead of allocating a
> new one and we should not free them.  That's it.

SLAB_NO_MERGE prevents both side of merging - when 1) creating the cache,
and when 2) another cache tries to create an alias from it.

Avoiding 1) makes sense, but is there a reason to prevent 2)?

If it's fine for other caches to merge into a cache with static
duration, then it's sufficient to update find_mergeable() to not attempt
creating an alias during cache creation if args->preallocated is
specified (instead of using SLAB_NO_MERGE).

-- 
Cheers,
Harry / Hyeonggon

> 	Note that slab-static.h is needed only in places that create
> such instances; all users need only slab.h (and they can be modular,
> unlike runtime_const-based approach).
> 
> 	That covers the instances that never get destroyed.  Quite a few
> fall into that category, but there's a major exception - anything in
> modules must be destroyed before the module gets removed.  For example,
> filesystems that have their inodes allocated from a private kmem_cache
> can't make use of that technics for their inode allocations, etc.
> 
> 	It's not that hard to deal with, but for now let's just ban
> including slab-static.h from modules.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>