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Re: [PATCH] xfs: reset buffer pointers before freeing them

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: reset buffer pointers before freeing them
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 12 Apr 2011 21:00:28 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110412000939.GC29358@xxxxxxxxxxxxx>
References: <1302491405-2290-1-git-send-email-david@xxxxxxxxxxxxx> <20110412000939.GC29358@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon, Apr 11, 2011 at 08:09:39PM -0400, Christoph Hellwig wrote:
> >                     error = xlog_bread_noalign(log, ealign, sectbb, bp);
> > -                   if (error)
> > -                           break;
> >  
> > -                   error = XFS_BUF_SET_PTR(bp, offset, bufblks);
> > +                   /* must reset buffer pointer even on error */
> > +                   error2 = XFS_BUF_SET_PTR(bp, offset, bufblks);
> 
> This seems to be incorrect both in the original and your version.
> From all that I can see XFS_BUF_SET_PTR wants a byte count while bufblks
> is a sector count, e.g. this should be:
> 
>                       error2 = XFS_BUF_SET_PTR(bp, offset, BBTOB(bufblks);

Yes, you are right. I missed that.

> While at I think this mess with the buffer virtual mapping, read into it
> and set it back mess needs to go into a single helper instead of beeing
> duplicated three times.  By having named arguments with proper types
> bugs like the existing one above should also be much more obvious to
> spot.

Ok, I'll try to come up with something sane.

> As a follow on patch to that I think we could also get rid of all the
> vmap manipultions entirely, and just specify and I/O offset and length
> manually.  The only thing required for that is a number of pages
> to skip at the beggining of the buffer from the log recovery code to
> _xfs_buf_ioapply, either by passing it down procedurally, or by adding
> a new filed to struct xfs_buf.  In fact just relaxing the b_offset
> semantics to allow offsets larger than a page only in the I/O path would
> do it, but I'm not sure that version helps long-term maintenance.

Passing the offset+len seems like the cleaner solution to me. I
don't like the idea of overloading existing fields just for the IO
path or adding new fields for one off uses, either. That can
probably wait for another day, though, because we've still got the
iclog buffer hackery to deal with.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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