Re: [PATCH 3/3] xfs: attach dquots before performing xfs_swap_extents

[Dave Chinner <david@fromorbit.com>]

On Mon, Nov 11, 2019 at 12:05:03AM -0800, Christoph Hellwig wrote:
> On Sun, Nov 10, 2019 at 05:18:34PM -0800, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Make sure we attach dquots to both inodes before swapping their extents.
> > This was found via manual code inspection by looking for places where we
> > could call xfs_trans_mod_dquot without dquots attached to inodes, and
> > confirmed by instrumenting the kernel and running xfs/328.
> 
> Looks good:
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 
> Btw, for  while I've been wondering if we could just get rid of the
> concepts of attached dquots.  With the radix-tree/xarray looks up
> are be fairly cheap, and could be done lockless using RCU.  So we could
> try to just kill the concept of attaching the dquot to the inode and
> just look it up once per operation, where operation preferally is
> something high-level like the actual file/inode operation and not a
> low-level thing inside xfs.

If the dquots are not attached to the inode, how would you pass the
3 dquots per inode down the stack to where they are actually used
inside the filesystem? I mean, we have to get the dquots attached to
the transaction so we can update them in xfs_trans_commit ->
xfs_trans_apply_dquot_deltas(), so somehow we'd have to get them
from the high level file/inode operations down to the XFS
transaction context. And things like writeback need dquots attached
for delayed allocation, so various aops would need to do dquot
lookups, too...

I can see the advantage of doing rcu dquot cache lookups in the xfs
context where we are attaching the dquots to the transaction rather
than attaching them to the inode, but I can't see how the "do it at
a high level" aspect of this would work....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com