linux-kernel - [PATCH v4] fs: hide names_cachep behind runtime access machinery

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Message-ID: <20251030105242.801528-1-mjguzik@gmail.com>
Date: Thu, 30 Oct 2025 11:52:40 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	torvalds@...ux-foundation.org,
	pfalcato@...e.de,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v4] fs: hide names_cachep behind runtime access machinery

The var is used twice for every path lookup, while the cache is
initialized early and stays valid for the duration.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

ACHTUNG WARNING POZOR UWAGA Блять: the namei cache can be used by
modules while the runtime machinery does not work with them. I did some
testing and ifdef MODULE seems to work around the probnlem, but perhaps
someone with build-fu could chime in? I verified with a hello world
module that this works fine, but maybe I missed a case.

v4:
- unbotch the diff below, apologies for the spam

v3:
- fix compilation failure on longarch as reported by kernel test robot,
  used their repro script to confirm

v2:
- ifdef on module usage -- the runtime thing does *not* work with modules
- patch up the section warn, thanks to Pedro for spotting what's up with
  the problem

Linus cc'ed as he added the runtime thing + dcache usage in the first place.

Per the above the machinery does not support kernel modules and I have
no interest in spending time to extend it.

I tried to add a compilation time warn should someone compile a module
with it, but there is no shared header so I decided to drop the matter.

Should someone(tm) make this work for modules I'm not going to protest.

Vast majority of actual usage is coming from core kernel, which *is*
getting the new treatment and I don't think the ifdef is particularly
nasty.

 fs/dcache.c                       |  3 +--
 include/asm-generic/vmlinux.lds.h |  3 ++-
 include/linux/fs.h                | 15 +++++++++++++--
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 035cccbc9276..ef83323276f0 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -35,8 +35,6 @@
 #include "internal.h"
 #include "mount.h"
 
-#include <asm/runtime-const.h>
-
 /*
  * Usage:
  * dcache->d_inode->i_lock protects:
@@ -3265,6 +3263,7 @@ void __init vfs_caches_init(void)
 {
 	names_cachep = kmem_cache_create_usercopy("names_cache", PATH_MAX, 0,
 			SLAB_HWCACHE_ALIGN|SLAB_PANIC, 0, PATH_MAX, NULL);
+	runtime_const_init(ptr, names_cachep);
 
 	dcache_init();
 	inode_init();
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index dcdbd962abd6..c7d85c80111c 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -939,7 +939,8 @@
 
 #define RUNTIME_CONST_VARIABLES						\
 		RUNTIME_CONST(shift, d_hash_shift)			\
-		RUNTIME_CONST(ptr, dentry_hashtable)
+		RUNTIME_CONST(ptr, dentry_hashtable)			\
+		RUNTIME_CONST(ptr, names_cachep)
 
 /* Alignment must be consistent with (kunit_suite *) in include/kunit/test.h */
 #define KUNIT_TABLE()							\
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 68c4a59ec8fb..cfaabd4824f2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -50,6 +50,8 @@
 #include <linux/unicode.h>
 
 #include <asm/byteorder.h>
+#include <asm/runtime-const.h>
+
 #include <uapi/linux/fs.h>
 
 struct backing_dev_info;
@@ -2960,8 +2962,17 @@ extern void __init vfs_caches_init(void);
 
 extern struct kmem_cache *names_cachep;
 
-#define __getname()		kmem_cache_alloc(names_cachep, GFP_KERNEL)
-#define __putname(name)		kmem_cache_free(names_cachep, (void *)(name))
+/*
+ * XXX The runtime_const machinery does not support modules at the moment.
+ */
+#ifdef MODULE
+#define __names_cachep		names_cachep
+#else
+#define __names_cachep		runtime_const_ptr(names_cachep)
+#endif
+
+#define __getname()		kmem_cache_alloc(__names_cachep, GFP_KERNEL)
+#define __putname(name)		kmem_cache_free(__names_cachep, (void *)(name))
 
 extern struct super_block *blockdev_superblock;
 static inline bool sb_is_blkdev_sb(struct super_block *sb)
-- 
2.34.1