[rfc][patch 2/2] mnt_want_write speedup 2

Nick Piggin
2008-12-19T06:18:07Z

This patch speeds up lmbench lat_mmap test by about another 2% after the
first patch.

Before:
avg = 462.286
std = 5.46106

After:
avg = 453.12
std = 9.58257

(50 runs of each, stddev gives a reasonable confidence)

It does this by introducing mnt_clone_write, which avoids some heavyweight
operations of mnt_want_write if called on a vfsmount which we know already
has a write count; and mnt_want_write_file, which can call mnt_clone_write
if the file is open for write.

After these two patches, mnt_want_write and mnt_drop_write go from 7% on
the profile down to 1.3% (including mnt_clone_write).

---
fs/file_table.c | 3 +--
fs/inode.c | 2 +-
fs/namespace.c | 38 ++++++++++++++++++++++++++++++++++++++
fs/open.c | 4 ++--
fs/xattr.c | 4 ++--
include/linux/mount.h | 2 ++
6 files changed, 46 insertions(+), 7 deletions(-)

Index: linux-2.6/fs/file_table.c
===================================================================
--- linux-2.6.orig/fs/file_table.c
+++ linux-2.6/fs/file_table.c
@@ -210,8 +210,7 @@ int init_file(struct file *file, struct
*/
if ((mode & FMODE_WRITE) && !special_file(dentry->d_inode->i_mode)) {
file_take_write(file);
- error = mnt_want_write(mnt);
- WARN_ON(error);
+ mnt_clone_write(mnt);
}
return error;
}
Index: linux-2.6/fs/inode.c
===================================================================
--- linux-2.6.orig/fs/inode.c
+++ linux-2.6/fs/inode.c
@@ -1249,7 +1249,7 @@ void file_update_time(struct file *file)
if (IS_NOCMTIME(inode))
return;

- err = mnt_want_write(file->f_path.mnt);
+ err = mnt_want_write_file(file->f_path.mnt, file);
if (err)
return;

Index: linux-2.6/fs/namespace.c
===================================================================
--- linux-2.6.orig/fs/namespace.c
+++ linux-2.6/fs/namespace.c
@@ -264,6 +264,44 @@ out:
EXPORT_SYMBOL_GPL(mnt_want_write);

/**
+ * mnt_clone_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * This is effectively like mnt_want_write, except
+ * it must only be used to take an extra write reference
+ * on a mountpoint that we already know has a write reference
+ * on it. This allows some optimisation.
+ *
+ * After finished, mnt_drop_write must be called as usual to
+ * drop the reference.
+ */
+void mnt_clone_write(struct vfsmount *mnt)
+{
+ preempt_disable();
+ inc_mnt_writers(mnt);
+ preempt_enable();
+}
+EXPORT_SYMBOL_GPL(mnt_clone_write);
+
+/**
+ * mnt_want_write_file - get write access to a file's mount
+ * @file: the file who's mount on which to take a write
+ *
+ * This is like mnt_want_write, but it takes a file and can
+ * do some optimisations if the file is open for write already
+ */
+int mnt_want_write_file(struct vfsmount *mnt, struct file *file)
+{
+ if (!(file->f_mode & FMODE_WRITE))
+ return mnt_want_write(mnt);
+ else {
+ mnt_clone_write(mnt);
+ return 0;
+ }
+}
+EXPORT_SYMBOL_GPL(mnt_want_write_file);
+
+/**
* mnt_drop_write - give up write access to a mount
* @mnt: the mount on which to give up write access
*
Index: linux-2.6/fs/open.c
===================================================================
--- linux-2.6.orig/fs/open.c
+++ linux-2.6/fs/open.c
@@ -597,7 +597,7 @@ asmlinkage long sys_fchmod(unsigned int

audit_inode(NULL, dentry);

- err = mnt_want_write(file->f_path.mnt);
+ err = mnt_want_write_file(file->f_path.mnt, file);
if (err)
goto out_putf;
mutex_lock(&inode->i_mutex);
@@ -748,7 +748,7 @@ asmlinkage long sys_fchown(unsigned int
if (!file)
goto out;

- error = mnt_want_write(file->f_path.mnt);
+ error = mnt_want_write_file(file->f_path.mnt, file);
if (error)
goto out_fput;
dentry = file->f_path.dentry;
Index: linux-2.6/fs/xattr.c
===================================================================
--- linux-2.6.orig/fs/xattr.c
+++ linux-2.6/fs/xattr.c
@@ -302,7 +302,7 @@ sys_fsetxattr(int fd, const char __user
return error;
dentry = f->f_path.dentry;
audit_inode(NULL, dentry);
- error = mnt_want_write(f->f_path.mnt);
+ error = mnt_want_write_file(f->f_path.mnt, f);
if (!error) {
error = setxattr(dentry, name, value, size, flags);
mnt_drop_write(f->f_path.mnt);
@@ -533,7 +533,7 @@ sys_fremovexattr(int fd, const char __us
return error;
dentry = f->f_path.dentry;
audit_inode(NULL, dentry);
- error = mnt_want_write(f->f_path.mnt);
+ error = mnt_want_write_file(f->f_path.mnt, f);
if (!error) {
error = removexattr(dentry, name);
mnt_drop_write(f->f_path.mnt);
Index: linux-2.6/include/linux/mount.h
===================================================================
--- linux-2.6.orig/include/linux/mount.h
+++ linux-2.6/include/linux/mount.h
@@ -90,6 +90,8 @@ static inline struct vfsmount *mntget(st
}

extern int mnt_want_write(struct vfsmount *mnt);
+extern int mnt_want_write_file(struct vfsmount *mnt, struct file *file);
+extern void mnt_clone_write(struct vfsmount *mnt);
extern void mnt_drop_write(struct vfsmount *mnt);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);