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SubjectRe: [PATCH 05/11] vfs: Add a function to lazily unmount all mounts from any dentry.
Miklos Szeredi <miklos@szeredi.hu> writes:

> On Sat, Feb 15, 2014 at 01:37:26PM -0800, Eric W. Biederman wrote:
>>
>> 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);
>> +
>> 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);
>
> Howabout a get_mountpoint(dentry) helper, that returns NULL if it turns out to
> be not a mountpoint? And, as an added bonus, you can drop the comment above as
> well.

I hate to admit it but that is a nice change. Especially as it allows
removing the d_mounpoint check inside of namespace_lock. I still need a
cheak d_mounpoint check outside of namespace lock but inside it can go.

The first time I looked at doing that I missed something and the change
looked too awkward to be worth implementing :(

Eric


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