[PATCH v2 19/35] ubifs: budget for inode in ubifs_dirty_inode if necessary

[Artem Bityutskiy]

On Thu, 2015-08-06 at 14:46 +0800, Dongsheng Yang wrote:
> On 08/05/2015 04:11 PM, Artem Bityutskiy wrote:
> > On Thu, 2015-07-30 at 13:48 +0800, Dongsheng Yang wrote:
> > > In ubifs, we have to do a budget for inode before marking
> > > it as dirty. But sometimes, we would call dirty_inode in vfs
> > > which will not do a budget for inode. In this case, we have
> > > to do a budget in ubifs_dirty_inode() by ourselvies.
> > > 
> > > Signed-off-by: Dongsheng Yang <yangds.fnst at cn.fujitsu.com>
> > 
> > Could you please explain some more the problem you are trying to 
> > solve.
> > Locking looks confusing and broken. It looks like what you are
> > expressing is that the 'ui_mutex' is optional, and this smells 
> > fishy.
> 
> Oh, yes, that's TRUE. This patch makes the locking broken. I am sorry
> about it.
> > 
> [...]
> > 
> > Please, try to explain what you want to achieve some more. I am not
> > sure I understand the end goal.
> 
> Okey, what I want here is to doing a budget for the inode in
> .dirty_inode called by vfs. Currently, the all work is under the full
> control of ubifs as the comment of @ui_mutex said. But the
> dquot_disable() is doing a dirty_inode() without asking ubifs is that
> allowed. So I want to do the budget in ubifs_dirty_inode() itself 
> here.But, that's INCORRECT. Yes, my bad. Thanx for your comment.
> 
> And I found another solution for it. To introduce a callback in quota
> to allow filesystem to dirty inode in dquot_disable(). I believe that
> works.

Yes, the high-level picture is this.

1. Dirtying and inode implies liability - the system becomes liable to
write it to the media.

2. Most file-systems have no problems with this, because there is
always space allocated for the inode.

3. Some file-systems like UBIFS may have no space for writing dirty
inodes. Often UBIFS can product this space, but it will involve forcing
write-back, and there are locking issues when doing write-back from the
write-back path.

4. So the approach UBIFS took is to allocate space in advance for every
dirty inode.

5. To do that, UBIFS requires VFS avoid dirtying inodes directly, but
instead, dirty them via the file-system. The file-system then has a
chance to do all the necessary things related to dirtying the inode.
The FS may even refuse to dirty the inode if in knows that it is
impossible to do (the FS is 100% full, for example).

So yes, if the generic quota code could dirty its inodes via the FS,
not directly, that would be great.

But please, check that the generic quota code can handle errors,
because ubifs_dirty_inode() may return -ENOSPC or other errors.

Artem.