Re: [PATCH v2] fs: hide names_cache behind runtime const machinery

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Dec 02, 2025 at 05:52:58AM +0000, Al Viro wrote:

> The delicate part is headers, indeed - we don't want to expose struct kmem_cache guts
> anywhere outside of mm/*, and not the entire mm/* either.  But that's not hard to
> deal with - see include/generate/bounds.h, include/generate/rq-offsets.h, etc.
> Exact same technics can be used to get sizeof(struct kmem_cache) calculated and
> put into generated header.  Then we get something like struct kmem_cache_store with
> the right size and alignment, and _that_ would be what the variables would be.
> With static inline struct kmem_cache *to_kmem_cache(struct kmem_cache_store *)
> returning a cast and e.g.
> 
> static inline void free_filename(struct __filename *p)
> {
>         kmem_cache_free(to_kmem_cache(&names_cache), p);
> }
> 
> as an example of use.
> 
> Anyway, for now I've applied your patch pretty much as-is; conversion of the
> sort described above can be done afterwards just fine.
> 

FWIW, the Kbuild side of that would be like this - not a lot of magic there:

diff --git a/Kbuild b/Kbuild
index 13324b4bbe23..eb985a6614eb 100644
--- a/Kbuild
+++ b/Kbuild
@@ -45,13 +45,24 @@ kernel/sched/rq-offsets.s: $(offsets-file)
 $(rq-offsets-file): kernel/sched/rq-offsets.s FORCE
 	$(call filechk,offsets,__RQ_OFFSETS_H__)
 
+# generate kmem_cache_size.h
+
+kmem_cache_size-file := include/generated/kmem_cache_size.h
+
+targets += mm/kmem_cache_size.s
+
+mm/kmem_cache_size.s: $(rq-offsets-file)
+
+$(kmem_cache_size-file): mm/kmem_cache_size.s FORCE
+	$(call filechk,offsets,__KMEM_CACHE_SIZE_H__)
+
 # Check for missing system calls
 
 quiet_cmd_syscalls = CALL    $<
       cmd_syscalls = $(CONFIG_SHELL) $< $(CC) $(c_flags) $(missing_syscalls_flags)
 
 PHONY += missing-syscalls
-missing-syscalls: scripts/checksyscalls.sh $(rq-offsets-file)
+missing-syscalls: scripts/checksyscalls.sh $(kmem_cache_size-file)
 	$(call cmd,syscalls)
 
 # Check the manual modification of atomic headers
diff --git a/include/linux/kmem_cache_store.h b/include/linux/kmem_cache_store.h
new file mode 100644
index 000000000000..4bd21480d3cf
--- /dev/null
+++ b/include/linux/kmem_cache_store.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_KMEM_CACHE_STORE_H
+#define __LINUX_KMEM_CACHE_STORE_H
+
+#include <generated/kmem_cache_size.h>
+
+/* same size and alignment as struct kmem_cache */
+struct kmem_cache_store {
+	unsigned char opaque[KMEM_CACHE_SIZE];
+} __attribute__((__aligned__(KMEM_CACHE_ALIGN)));
+
+struct kmem_cache;
+
+static inline struct kmem_cache *to_kmem_cache(struct kmem_cache_store *p)
+{
+	return (struct kmem_cache *)p;
+}
+#endif
diff --git a/mm/kmem_cache_size.c b/mm/kmem_cache_size.c
new file mode 100644
index 000000000000..52395b225aa1
--- /dev/null
+++ b/mm/kmem_cache_size.c
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generate definitions needed by the preprocessor.
+ * This code generates raw asm output which is post-processed
+ * to extract and format the required data.
+ */
+
+#define __GENERATING_KMEM_CACHE_SIZE_H
+/* Include headers that define the enum constants of interest */
+#include <linux/kbuild.h>
+#include "slab.h"
+
+int main(void)
+{
+	/* The enum constants to put into include/generated/bounds.h */
+	DEFINE(KMEM_CACHE_SIZE, sizeof(struct kmem_cache));
+	DEFINE(KMEM_CACHE_ALIGN, __alignof(struct kmem_cache));
+	/* End of constants */
+
+	return 0;
+}