[PATCH v3 1/3] proc: allow to mark /proc files permanent outside of fs/proc/

[Mateusz Guzik <mjguzik@gmail.com>]

From: Alexey Dobriyan <adobriyan@gmail.com>

Add proc_make_permanent() function to mark PDE as permanent to speed up
open/read/close (one alloc/free and lock/unlock less).

Enable it for built-in code and for compiled-in modules.
This function becomes nop magically in modular code.

		Note, note, note!

If built-in code creates and deletes PDEs dynamically (not in init
hook), then proc_make_permanent() must not be used.

It is intended for simple code:

	static int __init xxx_module_init(void)
	{
		g_pde = proc_create_single();
		proc_make_permanent(g_pde);
		return 0;
	}
	static void __exit xxx_module_exit(void)
	{
		remove_proc_entry(g_pde);
	}

If module is built-in then exit hook never executed and PDE is
permanent so it is OK to mark it as such.

If module is module then rmmod will yank PDE, but proc_make_permanent()
is nop and core /proc code will do everything right.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 fs/proc/generic.c       | 12 ++++++++++++
 fs/proc/internal.h      |  3 +++
 include/linux/proc_fs.h | 10 ++++++++++
 3 files changed, 25 insertions(+)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 3063080f3bb2..497561ee3848 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -845,3 +845,15 @@ ssize_t proc_simple_write(struct file *f, const char __user *ubuf, size_t size,
 	kfree(buf);
 	return ret == 0 ? size : ret;
 }
+
+/*
+ * Not exported to modules:
+ * modules' /proc files aren't permanent because modules aren't permanent.
+ */
+void impl_proc_make_permanent(struct proc_dir_entry *pde);
+void impl_proc_make_permanent(struct proc_dir_entry *pde)
+{
+	if (pde) {
+		pde_make_permanent(pde);
+	}
+}
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 64dc44832808..1edbabbdbc5d 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -79,8 +79,11 @@ static inline bool pde_is_permanent(const struct proc_dir_entry *pde)
 	return pde->flags & PROC_ENTRY_PERMANENT;
 }
 
+/* This is for builtin code, not even for modules which are compiled in. */
 static inline void pde_make_permanent(struct proc_dir_entry *pde)
 {
+	/* Ensure magic flag does something. */
+	static_assert(PROC_ENTRY_PERMANENT != 0);
 	pde->flags |= PROC_ENTRY_PERMANENT;
 }
 
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 19d1c5e5f335..dceccd27a234 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -248,4 +248,14 @@ static inline struct pid_namespace *proc_pid_ns(struct super_block *sb)
 
 bool proc_ns_file(const struct file *file);
 
+static inline void proc_make_permanent(struct proc_dir_entry *pde)
+{
+	/* Don't give matches to modules. */
+#if defined CONFIG_PROC_FS && !defined MODULE
+	/* This mess is created by defining "struct proc_dir_entry" elsewhere. */
+	void impl_proc_make_permanent(struct proc_dir_entry *pde);
+	impl_proc_make_permanent(pde);
+#endif
+}
+
 #endif /* _LINUX_PROC_FS_H */
-- 
2.48.1