Re: UBIFS quota support

[Richard Weinberger <richard@nod.at>]

Sascha,

Am Mittwoch, 23. Januar 2019, 10:43:05 CET schrieb Sascha Hauer:
> On Wed, Jan 23, 2019 at 12:07:12AM +0100, Richard Weinberger wrote:
> > On Thu, Jan 10, 2019 at 12:45 PM Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > >
> > > Hi all,
> > >
> > > I'm currently working on resurrecting the UBIFS quota patches posted back in
> > > 2015 by Dongsheng Yang, last posted here:
> > >
> > > http://lists.infradead.org/pipermail/linux-mtd/2015-September/061812.html
> > >
> > > First of all I think work stopped there, there is no newer UBIFS quota
> > > support I am missing, right?
> > >
> > > One problem with this series was that the quotactl systemcall expects a
> > > path to a block device. UBIFS doesn't work on a block device but on a
> > > character device instead.
> > > The solution in this series was to pass the path to the cdev in
> > > quotactl.  A struct cdev * member was added to struct super_block which
> > > was used to identify the superblock for a given cdev. This approach was
> > > rejected by Christoph ("I don't think the cdev has any business in core
> > > VFS code.").  Apart from that UBIFS can not only be mounted with a path
> > > to the character device (mount -t ubifs /dev/ubix_y /mnt) but also in
> > > the form ubix:volname (mount -t ubifs ubix:volname /mnt) in which case
> > > userspace doesn't have any valid path it could pass in quotactl.
> > >
> > > An idea out of this would be to allow to pass the mountpoint instead of
> > > the path to the block device in quotactl which would work with nfs or
> > > even tmpfs aswell. Would that be acceptable? Any other ideas?
> > 
> > *kind ping*
> > 
> > Jan, another thing Sascha and I are not sure about, what are the consistency
> > constraints of the quota file?
> > If I read the code correctly, quota just writes to the quota file and
> > assumes that
> > the file system makes sure about consistency. Either by fsckfixing the quota
> > file or having a data journal for the quota file.
> > In case of UBIFS where we have a data journal this should be doable.
> > Is it okay when the quota file has S_SYNC set?
> 
> S_SYNC won't help us. We need to make sure that a change of an inode and
> the corresponding update to the quota file is done atomically. Otherwise
> it may happen that we only change the size of an inode, but miss the
> corresponding quota updates, or depending on the implementation, maybe
> the other way round.

This is why I said yesterday you need to touch the UBIFS journal replay code too.
So, S_SYNC is not to keep the quota file consistent with UBIFS' state, it is
to make sure we don't lose quota updates.
Skipping S_SYNC is also possible, but then you need to fix up even more stuff
in the replay code.

Thanks,
//richard