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SubjectRe: [PATCH 05/11] vfs: Add a function to lazily unmount all mounts from any dentry.
ebiederm@xmission.com (Eric W. Biederman) writes:

> v2: Always drop the lock when exiting early.
> v3: Make detach_mounts robust about freeing several
> mounts on the same mountpoint at one time, and remove
> the unneeded mnt_list list test.
> v4: Document the purpose of detach_mounts and why new_mountpoint is
> safe to call.
>
> Signed-off-by: Eric W. Biederman <ebiederman@twitter.com>
> ---
> fs/mount.h | 2 ++
> fs/namespace.c | 39 +++++++++++++++++++++++++++++++++++++++
> 2 files changed, 41 insertions(+), 0 deletions(-)
>
> diff --git a/fs/mount.h b/fs/mount.h
> index 50a72d46e7a6..2b470f34e665 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -84,6 +84,8 @@ extern struct mount *__lookup_mnt_last(struct vfsmount *, struct dentry *);
>
> extern bool legitimize_mnt(struct vfsmount *, unsigned);
>
> +extern void detach_mounts(struct dentry *dentry);

After Linus's rant about performance I took a second look, and lo and
behold there is a significant optimization oppportunity missed here.

This should almost certainly be:

extern void __detach_mounts(struct dentry *dentry);

static inline void detach_mounts(struct dentry *dentry)
{
if (unlikely(d_mountpoint(dentry)))
return;
__detach_mounts(dentry);
}

Just so I don't take the namespace lock when not necessary.

Although wether this should be inline or at the start of detach_mounts
and out of line I don't know. Sigh. I will have to play with this
detail a bit more.

Eric



> static inline void get_mnt_ns(struct mnt_namespace *ns)
> {
> atomic_inc(&ns->count);
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 33db9e95bd5c..7abbf722ce18 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1359,6 +1359,45 @@ static int do_umount(struct mount *mnt, int flags)
> return retval;
> }
>
> +/*
> + * detach_mounts - lazily unmount all mounts on the specified dentry
> + *
> + * During unlink, rmdir, and d_drop it is possible to loose the path
> + * to an existing mountpoint, and wind up leaking the mount.
> + * detach_mounts allows lazily unmounting those mounts instead of
> + * leaking them.
> + *
> + * The caller may hold dentry->d_inode->i_mutex.
> + */
> +void detach_mounts(struct dentry *dentry)
> +{
> + struct mountpoint *mp;
> + struct mount *mnt;
> +
> + namespace_lock();
> + if (!d_mountpoint(dentry))
> + goto out_unlock;
> +
> + /*
> + * The namespace lock and d_mountpoint being set guarantees
> + * that new_mountpoint will just be a lookup of the existing
> + * mountpoint structure.
> + */
> + mp = new_mountpoint(dentry);
> + if (IS_ERR(mp))
> + goto out_unlock;
> +
> + lock_mount_hash();
> + while (!list_empty(&mp->m_list)) {
> + mnt = list_first_entry(&mp->m_list, struct mount, mnt_mp_list);
> + umount_tree(mnt, 1);
> + }
> + unlock_mount_hash();
> + put_mountpoint(mp);
> +out_unlock:
> + namespace_unlock();
> +}
> +
> /*
> * Is the caller allowed to modify his namespace?
> */


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