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Subject: Re: [PATCH 2/2 v2] xfs_repair: new secondary superblock search
	method] xfs_repair: new secondary superblock search method
References: <1455068099-26992-1-git-send-email-billodo@redhat.com>
	<56BAC150.7070808@sandeen.net> <20160210154300.GA9472@redhat.com>
From: Eric Sandeen <sandeen@sandeen.net>
Message-ID: <56BB5C67.6040906@sandeen.net>
Date: Wed, 10 Feb 2016 09:51:03 -0600
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To: Bill O'Donnell <billodo@redhat.com>
Cc: xfs@oss.sgi.com

On 2/10/16 9:43 AM, Bill O'Donnell wrote:
>> I'd make __find_secondary_sb() take (sb, start, skip) i.e. send in
>> > this:
>> > 
>>> > > +	retval = __find_secondary_sb(rsb, agsize, agsize);
>>> > > +	if (!retval)  {
>>> > > +		/*
>>> > > +		 * fallback: use minimum agsize for skipsize
>>> > > +		 */
>>> > > +		retval = __find_secondary_sb(rsb, XFS_AG_MIN_BYTES, BSIZE);
>>> > > +	}
>> > 
>> > and the function is something like:
>> > 
>> > static int
>> > __find_secondary_sb(
>> >         xfs_sb_t	*rsb,
>> > 	xfs_off_t	start,
>> >         xfs_off_t	skip)
>> > 
>> > {
>> > 
>> > ...
>> > 
>> >         for (done = 0, off = start; !done ; off += skip)  {
>> > ...
>> >                if (lseek64(x.dfd, off, SEEK_SET) != off)
>> >                         done = 1;
>> > 
>> >                if (!done && (read(x.dfd, sb, BSIZE)) <= 0)
>> >                         done = 1;
> But, bsize is used here:
> ...
>      	    * check the buffer 512 bytes at a time since
> 	    * we don't know how big the sectors really are.
> 	    */
> 	    for (i = 0; !done && i < bsize; i += BBSIZE)  {
> ...
> so, don't we still need to populate bsize? Or does it make more
> sense to just use BBSIZE in the conditional, ala:
>             for (i = 0; !done && i < BBSIZE; i += BBSIZE) 

Oh, right, sorry.  yes, keep the assignment:

                if (!done && (bsize = read(x.dfd, sb, BSIZE)) <= 0)  {

That handles a short read at the end; we ask for BSIZE, actually
read bsize, and then we need to iterate over what actually got read
(bsize).

BSIZE, bsize ... clear as mud.  :(

-Eric

>> > 
>> > 
>> > because you really can't deduce both the starting point and the skip-ahead
>> > size from just one parameter.
> Agreed.
> Thanks for your thorough reviews :)
> Bill
> 

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