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    Subject[patch 3/5] 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).

    Cc: Dave Hansen <haveblue@us.ibm.com>
    Signed-off-by: Nick Piggin <npiggin@suse.de>
    ---
    fs/file_table.c | 2 +-
    fs/inode.c | 2 +-
    fs/namespace.c | 40 ++++++++++++++++++++++++++++++++++++++++
    fs/open.c | 4 ++--
    fs/xattr.c | 4 ++--
    include/linux/mount.h | 4 ++++
    6 files changed, 50 insertions(+), 6 deletions(-)

    Index: linux-2.6/fs/file_table.c
    ===================================================================
    --- linux-2.6.orig/fs/file_table.c
    +++ linux-2.6/fs/file_table.c
    @@ -214,7 +214,7 @@ 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);
    + error = mnt_clone_write(mnt);
    WARN_ON(error);
    }
    return error;
    Index: linux-2.6/fs/inode.c
    ===================================================================
    --- linux-2.6.orig/fs/inode.c
    +++ linux-2.6/fs/inode.c
    @@ -1401,7 +1401,7 @@ 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
    @@ -265,6 +265,46 @@ 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.
    + */
    +int mnt_clone_write(struct vfsmount *mnt)
    +{
    + /* superblock may be r/o */
    + if (__mnt_is_readonly(mnt))
    + return -EROFS;
    + preempt_disable();
    + inc_mnt_writers(mnt);
    + preempt_enable();
    + return 0;
    +}
    +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
    + return mnt_clone_write(mnt);
    +}
    +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
    @@ -612,7 +612,7 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd

    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);
    @@ -761,7 +761,7 @@ SYSCALL_DEFINE3(fchown, unsigned int, fd
    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
    @@ -297,7 +297,7 @@ SYSCALL_DEFINE5(fsetxattr, int, fd, cons
    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);
    @@ -524,7 +524,7 @@ SYSCALL_DEFINE2(fremovexattr, int, fd, c
    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
    @@ -88,7 +88,11 @@ static inline struct vfsmount *mntget(st
    return mnt;
    }

    +struct file; /* forward dec */
    +
    extern int mnt_want_write(struct vfsmount *mnt);
    +extern int mnt_want_write_file(struct vfsmount *mnt, struct file *file);
    +extern int 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);



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