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

[Christoph Hellwig <hch@infradead.org>]

On Wed, Nov 13, 2019 at 09:14:48AM +1100, Dave Chinner wrote:
> 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...

My prime idea was to attach them to the transaction and keep them
over transaction roles.  Then see what is left and probably use an
on-stack struct containing three dquots.  At that point I know if
that idea was feasible, because if we have too many deep callstacks
where we need to pass that struct it obviously isn't.

> 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....

Most of our ops really just have one transaction / set of rolled
over permanent transactions, because if they didn't they wouldn't
be atomic..