fs: introduce mnt_clone_write

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).

[AV: mnt_want_write_file() should take file alone and derive mnt from it;
not only all callers have that form, but that's the only mnt about which
we know that it's already held for write if file is opened for write]

Cc: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/xattr.c b/fs/xattr.c
index d51b8f9db921..1c3d0af59ddf 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -297,7 +297,7 @@ SYSCALL_DEFINE5(fsetxattr, int, fd, const char __user *, name,
                 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);
         if (!error) {
                 error = setxattr(dentry, name, value, size, flags);
                 mnt_drop_write(f->f_path.mnt);
@@ -524,7 +524,7 @@ SYSCALL_DEFINE2(fremovexattr, int, fd, const char __user *, name)
                 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);
         if (!error) {
                 error = removexattr(dentry, name);
                 mnt_drop_write(f->f_path.mnt);