On 7/29/2015 10:06, NeilBrown wrote:
> On Mon, 27 Jul 2015 11:08:32 +0800 Kinglong Mee <kinglongmee@xxxxxxxxx>
> wrote:
>
>> New helper legitimize_mntget for getting a mnt without setting
>> MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED, otherwise return NULL.
>>
>> v8, same as v6
>>
>> Signed-off-by: Kinglong Mee <kinglongmee@xxxxxxxxx>
>> ---
>> fs/namespace.c | 19 +++++++++++++++++++
>> include/linux/mount.h | 1 +
>> 2 files changed, 20 insertions(+)
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 2b8aa15..842cf57 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -1153,6 +1153,25 @@ struct vfsmount *mntget(struct vfsmount *mnt)
>> }
>> EXPORT_SYMBOL(mntget);
>>
>> +struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt)
>> +{
>> + struct mount *mnt;
>> +
>> + if (vfsmnt == NULL)
>> + return NULL;
>> +
>> + read_seqlock_excl(&mount_lock);
>> + mnt = real_mount(vfsmnt);
>> + if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED))
>> + vfsmnt = NULL;
>> + else
>> + mnt_add_count(mnt, 1);
>> + read_sequnlock_excl(&mount_lock);
>> +
>> + return vfsmnt;
>> +}
>> +EXPORT_SYMBOL(legitimize_mntget);
>> +
>> struct vfsmount *mnt_clone_internal(struct path *path)
>> {
>> struct mount *p;
>> diff --git a/include/linux/mount.h b/include/linux/mount.h
>> index f822c3c..8ae9dc0 100644
>> --- a/include/linux/mount.h
>> +++ b/include/linux/mount.h
>> @@ -79,6 +79,7 @@ extern void mnt_drop_write(struct vfsmount *mnt);
>> extern void mnt_drop_write_file(struct file *file);
>> extern void mntput(struct vfsmount *mnt);
>> extern struct vfsmount *mntget(struct vfsmount *mnt);
>> +extern struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt);
>> extern struct vfsmount *mnt_clone_internal(struct path *path);
>> extern int __mnt_is_readonly(struct vfsmount *mnt);
>>
>
> It is unfortunate that we seem to have to take the mount_lock global
> lock on every nfs request. I wonder if we can avoid that....
>
> What if we did:
>
> seq = 0;
> retry:
> read_seqbegin_or_lock(&mount_lock, &seq);
> if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED))
> vfsmnt = NULL;
> else if (need_seqretry(&mount_lock, seq);
> goto retry;
> else {
> mnt_add_count(&mnt, 1);
> if (need_seqretry(&mount_lock, seq) ||
> vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT |
> MNT_DOOMED)) {
> mnt_add_count(&mnt, -1);
> goto retry;
> }
> }
> done_seqretry(&mount_lock, seq);
I don't really understand the seqlock, I will have a check about it.
>
>
> Is there any risk from having a temporarily elevated mnt_count there?
> I can't see one, but there is clearly some complexity in managing that
> count.
Yes, it's more complex than the above patch.
But, if it can avoid taking the mount_lock every nfs request, I'd like it.
I will test it later.
thanks,
Kinglong Mee
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
