On Fri, 30 Jan 2015 10:02:26 +0800, Qu Wenruo wrote:
>
> -------- Original Message --------
> Subject: Re: [PATCH RESEND v4 6/8] vfs: Add get_vfsmount_sb() function to get
> vfsmount from a given sb.
> From: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
> To: Miao Xie <miaoxie@xxxxxxxxxx>, linux-btrfs@xxxxxxxxxxxxxxx
> Date: 2015年01月30日 09:44
>>
>> -------- Original Message --------
>> Subject: Re: [PATCH RESEND v4 6/8] vfs: Add get_vfsmount_sb() function to get
>> vfsmount from a given sb.
>> From: Miao Xie <miaoxie@xxxxxxxxxx>
>> To: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>, <linux-btrfs@xxxxxxxxxxxxxxx>
>> Date: 2015年01月30日 08:52
>>> On Thu, 29 Jan 2015 10:24:39 +0800, Qu Wenruo wrote:
>>>> There are sysfs interfaces in some fs, only btrfs yet, which will modify
>>>> on-disk data.
>>>> Unlike normal file operation routine we can use mnt_want_write_file() to
>>>> protect the operation, change through sysfs won't to be binded to any file
>>>> in the filesystem.
>>>> So we can only extract the first vfsmount of a superblock and pass it to
>>>> mnt_want_write() to do the protection.
>>> This method is wrong, becasue one fs may be mounted on the multi places
>>> at the same time, someone is R/O, someone is R/W, you may get a R/O and
>>> fail to get the write permission.
>> This shouldn't happen. If someone is ro, the whole fs should be ro, right?
>> You can mount a device which is already mounted as rw to other point as ro,
>> and remount a mount point to ro will also cause all other mount point to ro.
>>
>> So I didn't see the problem here.
>>>
>>> I think you do label/feature change by sysfs interface by the following way
>>>
>>> btrfs_sysfs_change_XXXX()
>>> {
>>> /* Use trylock to avoid the race with umount */
>>> if(!mutex_trylock(&sb->s_umount))
>>> return -EBUSY;
>>>
>>> check R/O and FREEZE
>>>
>>> mutex_unlock(&sb->s_umount);
>>> }
>> This looks better since it not introduce changes to VFS.
>>
>> Thanks,
>> Qu
> Oh, wait a second, this one leads to the old problem and old solution.
>
> If we hold s_umount mutex, we must do freeze check and can't start transaction
> since it will deadlock.
>
> And for freeze check, we must use sb_try_start_intwrite() to hold the freeze
> lock and then add a new
> btrfs_start_transaction_freeze() which will not call sb_start_write()...
>
> Oh this seems so similar, v2 or v3 version RFC patch?
> So still goes to the old method?
No. Just check R/O and RREEZE, if failed, go out. if the check pass,
we start_transaction. Because we do it in s_umount lock, no one can
change fs to R/O or FREEZE.
Maybe the above description is not so clear, explain it again.
btrfs_sysfs_change_XXXX()
{
/* Use trylock to avoid the race with umount */
if(!mutex_trylock(&sb->s_umount))
return -EBUSY;
if (fs is R/O or FREEZED) {
mutex_unlock(&sb->s_umount);
return -EACCES;
}
btrfs_start_transaction()
change label/feature
btrfs_commit_transaction()
mutex_unlock(&sb->s_umount);
}
Thanks
Miao
>
> Thanks,
> Qu
>>>
>>> Thanks
>>> Miao
>>>
>>>> Cc: linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>
>>>> Signed-off-by: Qu Wenruo <quwenruo@xxxxxxxxxxxxxx>
>>>> ---
>>>> fs/namespace.c | 25 +++++++++++++++++++++++++
>>>> include/linux/mount.h | 1 +
>>>> 2 files changed, 26 insertions(+)
>>>>
>>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>>> index cd1e968..5a16a62 100644
>>>> --- a/fs/namespace.c
>>>> +++ b/fs/namespace.c
>>>> @@ -1105,6 +1105,31 @@ struct vfsmount *mntget(struct vfsmount *mnt)
>>>> }
>>>> EXPORT_SYMBOL(mntget);
>>>> +/*
>>>> + * get a vfsmount from a given sb
>>>> + *
>>>> + * This is especially used for case where change fs' sysfs interface
>>>> + * will lead to a write, e.g. Change label through sysfs in btrfs.
>>>> + * So vfs can get a vfsmount and then use mnt_want_write() to protect.
>>>> + */
>>>> +struct vfsmount *get_vfsmount_sb(struct super_block *sb)
>>>> +{
>>>> + struct vfsmount *ret_vfs = NULL;
>>>> + struct mount *mnt;
>>>> + int ret = 0;
>>>> +
>>>> + lock_mount_hash();
>>>> + if (list_empty(&sb->s_mounts))
>>>> + goto out;
>>>> + mnt = list_entry(sb->s_mounts.next, struct mount, mnt_instance);
>>>> + ret_vfs = &mnt->mnt;
>>>> + ret_vfs = mntget(ret_vfs);
>>>> +out:
>>>> + unlock_mount_hash();
>>>> + return ret_vfs;
>>>> +}
>>>> +EXPORT_SYMBOL(get_vfsmount_sb);
>>>> +
>>>> struct vfsmount *mnt_clone_internal(struct path *path)
>>>> {
>>>> struct mount *p;
>>>> diff --git a/include/linux/mount.h b/include/linux/mount.h
>>>> index c2c561d..cf1b0f5 100644
>>>> --- a/include/linux/mount.h
>>>> +++ b/include/linux/mount.h
>>>> @@ -79,6 +79,7 @@ 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 *mnt_clone_internal(struct path *path);
>>>> +extern struct vfsmount *get_vfsmount_sb(struct super_block *sb);
>>>> extern int __mnt_is_readonly(struct vfsmount *mnt);
>>>> struct path;
>>>>
>>
>
> .
>
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