Re: [PATCH] reduce large do_mount stack usage with noinlines

[Eric Sandeen]

(updated with comments about the noinlines, and a fix for
 an >80 char line)

do_mount() uses a whopping 616 bytes of stack on x86_64 in 
2.6.24-mm1, largely thanks to gcc inlining the various helper 
functions.

noinlining these can slim it down a lot; on my box this patch
gets it down to 168, which is mostly the struct nameidata nd;
left on the stack.

These functions are called only as do_mount() helpers;
none of them should be in any path that would see a performance
benefit from inlining...

Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
---

Index: linux-2.6.24-mm1/fs/namespace.c
===================================================================
--- linux-2.6.24-mm1.orig/fs/namespace.c
+++ linux-2.6.24-mm1/fs/namespace.c
@@ -1295,8 +1295,9 @@ out_unlock:
 
 /*
  * recursively change the type of the mountpoint.
+ * noinline this do_mount helper to save do_mount stack space.
  */
-static int do_change_type(struct nameidata *nd, int flag)
+static noinline int do_change_type(struct nameidata *nd, int flag)
 {
 	struct vfsmount *m, *mnt = nd->path.mnt;
 	int recurse = flag & MS_REC;
@@ -1319,8 +1320,10 @@ static int do_change_type(struct nameida
 
 /*
  * do loopback mount.
+ * noinline this do_mount helper to save do_mount stack space.
  */
-static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
+static noinline int do_loopback(struct nameidata *nd, char *old_name,
+				int recurse)
 {
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
@@ -1386,8 +1389,9 @@ static int change_mount_flags(struct vfs
  * change filesystem flags. dir should be a physical root of filesystem.
  * If you've mounted a non-root directory somewhere and want to do remount
  * on it - tough luck.
+ * noinline this do_mount helper to save do_mount stack space.
  */
-static int do_remount(struct nameidata *nd, int flags, int mnt_flags,
+static noinline int do_remount(struct nameidata *nd, int flags, int mnt_flags,
 		      void *data)
 {
 	int err;
@@ -1425,7 +1429,10 @@ static inline int tree_contains_unbindab
 	return 0;
 }
 
-static int do_move_mount(struct nameidata *nd, char *old_name)
+/*
+ * noinline this do_mount helper to save do_mount stack space.
+ */
+static noinline int do_move_mount(struct nameidata *nd, char *old_name)
 {
 	struct nameidata old_nd, parent_nd;
 	struct vfsmount *p;
@@ -1504,8 +1511,9 @@ out:
 /*
  * create a new mount for userspace and request it to be added into the
  * namespace's tree
+ * noinline this do_mount helper to save do_mount stack space.
  */
-static int do_new_mount(struct nameidata *nd, char *type, int flags,
+static noinline int do_new_mount(struct nameidata *nd, char *type, int flags,
 			int mnt_flags, char *name, void *data)
 {
 	struct vfsmount *mnt;

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