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Re: [PATCH 08/18] xfs: create helper to manage record overlap for sparse

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 08/18] xfs: create helper to manage record overlap for sparse inode chunks
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 29 Jul 2014 11:10:18 -0400
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140729000733.GG26465@dastard>
References: <1406211788-63206-1-git-send-email-bfoster@xxxxxxxxxx> <1406211788-63206-9-git-send-email-bfoster@xxxxxxxxxx> <20140724224112.GR20518@dastard> <20140728161925.GD59542@xxxxxxxxxxxxxxx> <20140729000733.GG26465@dastard>
User-agent: Mutt/1.5.23 (2014-03-12)
On Tue, Jul 29, 2014 at 10:07:33AM +1000, Dave Chinner wrote:
> On Mon, Jul 28, 2014 at 12:19:25PM -0400, Brian Foster wrote:
> > On Fri, Jul 25, 2014 at 08:41:12AM +1000, Dave Chinner wrote:
> > > On Thu, Jul 24, 2014 at 10:22:58AM -0400, Brian Foster wrote:
> > > > +       struct xfs_btree_cur            *cur;
> > > > +       struct xfs_agi                  *agi = XFS_BUF_TO_AGI(agbp);
> > > > +       xfs_agnumber_t                  agno = 
> > > > be32_to_cpu(agi->agi_seqno);
> > > > +       xfs_agino_t                     previno;
> > > > +       int                             error;
> > > > +       int                             i;
> > > > +       struct xfs_inobt_rec_incore     rec;
> > > > +
> > > > +       orec->ir_startino = NULLAGINO;
> > > > +
> > > > +       cur = xfs_inobt_init_cursor(mp, tp, agbp, agno, btnum);
> > > > +
> > > > +       previno = newino + count - XFS_INODES_PER_CHUNK;
> > > > +       error = xfs_inobt_lookup(cur, previno, XFS_LOOKUP_GE, &i);
> > > 
> > > You want XFS_LOOKUP_EQ, yes? i.e. XFS_LOOKUP_GE won't fail if the
> > > exact record for the inode chunk does not exist - it will return the
> > > next one in the btree.
> > > 
> > 
> > Assuming variable sparse chunk granularity,
> 
> Isn't the granularity fixed for the specific filesystem
> configuration as part of the on-disk format?
> 

Sort of, but I'm thinking of that as a limitation of the imap code and
such. I'd like to avoid introducing more of such assumptions where
possible in the implementation. That's what I meant before about not
explicitly encoding it. I wanted to use the cluster size (now the
"spinoalignmt") only in the few places that needed the allocation size
and let the rest of the code make no assumptions and work against the
minimum granularity defined by the on-disk format (i.e., inodes per
holemask bit, inodes per record).

The only reason I had to base the sparse alloc. granularity on the
cluster size is so I don't have to go through and figure out how to fix
that inode buffer code as a dependency to get a basic mechanism working.

There's also the scenario where if the granularity can end up small
enough, I'm not sure we can reliably calculate the starting inode of a
record (unless we make changes in the allocation path). TBH, even if we
could, I'd rather keep the code flexible and warn/assert/error on the
failed assumption with more information.

> > I don't really know the
> > start ino of the record that potentially covers the new inode chunk.
> > Given that, we use the smallest possible start ino that could include
> > this chunk and search forward from there. As you've noted below, I
> > wasn't relying on failure here to detect the scenario where there is no
> > existing record.
> 
> Ok, that's not how I thought the code was attempting to implement
> the "has record" check. My mistake - a comment explaining how the
> match is supposed to work would be helpful, I think.
> 

Indeed, I'll add a comment with some context.

> However, with that in mind, why do you even bother calculating at
> "previno"?  If you want the chunk that the "newino" lies in, then
> by definition it's going to be the first record at an equal or
> lower start inode number than newino.  i.e.:
> 
>       xfs_inobt_lookup(cur, newino, XFS_LOOKUP_LE, &i);
> 
> Will return either:
> 
>       - a match with startino <= newino < startino + XFS_INODES_PER_CHUNK
>       - a match with startino + XFS_INODES_PER_CHUNK <= newino
>       - a failure due to no record.
> 
> i.e. the first case is the chunk record we want, the others are
> "does not exist" failures. We don't need to calculate the "previno"
> at all.
> 

Yeah, that might be nicer. I'll try the search in the other direction.
Thanks.

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
> 
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