Message-ID: <20260110040217.1927971-1-viro@zeniv.linux.org.uk>
Date: Sat, 10 Jan 2026 04:02:02 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: linux-mm@...ck.org
Cc: Vlastimil Babka <vbabka@...e.cz>,
	Harry Yoo <harry.yoo@...cle.com>,
	linux-fsdevel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	Mateusz Guzik <mguzik@...il.com>,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH 00/15] kmem_cache instances with static storage duration

        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.

	A set of helpers parallel to kmem_cache_create() and friends
(kmem_cache_setup(), etc.) is provided in the same linux/slab-static.h;
generally, conversion affects only a few lines.

	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.  Note that
unlike runtime_constant-based approach, cache _uses_ in a module are
fine - if kmem_cache_opaque instance is exported, its address is available
to modules without any problems.  It's caches _created_ in a module
that offer an extra twist.

	Teaching kmem_cache_destroy() to skip actual freeing of given
kmem_cache instance is trivial; the problem is that kmem_cache_destroy()
may overlap with sysfs access to attributes of that cache.  In that
case kmem_cache_destroy() may return before the instance gets freed -
freeing (from slab_kmem_cache_release()) happens when the refcount of
embedded kobject drops to zero.  That's fine, since all references
to data structures in module's memory are already gone by the time
kmem_cache_destroy() returns.  That, however, relies upon the struct
kmem_cache itself not being in module's memory; getting it unmapped
before slab_kmem_cache_release() has run needs to be avoided.

	It's not hard to deal with, though.  We need to make sure that
instance in a module will get to slab_kmem_cache_release() before the
module data gets freed.  That's only a problem on sysfs setups -
otherwise it'll definitely be finished before kmem_cache_destroy()
returns.

	Note that modules themselves have sysfs-exposed attributes,
so a similar problem already exists there.  That's dealt with by
having mod_sysfs_teardown() wait for refcount of module->mkobj.kobj
reaching zero.  Let's make use of that - have static-duration-in-module
kmem_cache instances grab a reference to that kobject upon setup and
drop it in the end of slab_kmem_cache_release().

	Let setup helpers store the kobjetct to be pinned in
kmem_cache_args->owner (for preallocated; if somebody manually sets it
for non-preallocated case, it'll be ignored).  That would be
&THIS_MODULE->mkobj.kobj for a module and NULL in built-in.

	If sysfs is enabled and we are dealing with preallocated instance,
let create_cache() grab and stash that reference in kmem_cache->owner
and let slab_kmem_cache_release() drop it instead of freeing kmem_cache
instance.


	Costs:
* a bit (SLAB_PREALLOCATED) is stolen from slab_flags_t
* such caches can't be merged.  If you want them mergable, don't use that
technics.
* you can't do kmem_cache_setup()/kmem_cache_destroy()/kmem_cache_setup()
on the same instance.  Just don't do that.

Al Viro (15):
  static kmem_cache instances for core caches
  allow static-duration kmem_cache in modules
  make mnt_cache static-duration
  turn thread_cache static-duration
  turn signal_cache static-duration
  turn bh_cachep static-duration
  turn dentry_cache static-duration
  turn files_cachep static-duration
  make filp and bfilp caches static-duration
  turn sighand_cache static-duration
  turn mm_cachep static-duration
  turn task_struct_cachep static-duration
  turn fs_cachep static-duration
  turn inode_cachep static-duration
  turn ufs_inode_cache static-duration

 Kbuild                            | 13 +++++-
 fs/buffer.c                       |  6 ++-
 fs/dcache.c                       |  8 ++--
 fs/file_table.c                   | 32 +++++++-------
 fs/inode.c                        |  6 ++-
 fs/namespace.c                    |  6 ++-
 fs/ufs/super.c                    |  9 ++--
 include/asm-generic/vmlinux.lds.h |  3 +-
 include/linux/fdtable.h           |  3 +-
 include/linux/fs_struct.h         |  3 +-
 include/linux/signal.h            |  3 +-
 include/linux/slab-static.h       | 69 +++++++++++++++++++++++++++++++
 include/linux/slab.h              | 11 +++++
 kernel/fork.c                     | 37 ++++++++++-------
 mm/kmem_cache_size.c              | 20 +++++++++
 mm/slab.h                         |  1 +
 mm/slab_common.c                  | 44 +++++++++++++-------
 mm/slub.c                         |  7 ++++
 18 files changed, 214 insertions(+), 67 deletions(-)
 create mode 100644 include/linux/slab-static.h
 create mode 100644 mm/kmem_cache_size.c

-- 
2.47.3

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