linux-kernel - Re: [PATCH] REQ-flags to/from BIO-flags bugfix

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Date:	Thu, 13 Dec 2007 16:36:21 +1100
From:	David Chinner <dgc@....com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Boaz Harrosh <bharrosh@...asas.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gabriel C <nix.or.die@...glemail.com>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH] REQ-flags to/from BIO-flags bugfix

On Wed, Dec 12, 2007 at 08:54:07AM -0700, Matthew Wilcox wrote:
> On Wed, Dec 12, 2007 at 08:18:14AM -0700, Matthew Wilcox wrote:
> > I don't know whether BIO_RW_BARRIER is __REQ_SOFTBARRIER or
> > __REQ_HARDBARRIER, so I didn't include that in this patch.  There also
> > doesn't seem to be a __REQ equivalent to BIO_RW_AHEAD, but we can do
> > the other four bits (and leave gaps for those two).
> 
> Hm.  BIO_RW_AHEAD seems unused:
> 
> willy@...eydew:~/kernel/linux-2.6$ grep -r BIO_RW_AHEAD *
> block/blktrace.c:       (((rw) & (1 << BIO_RW_AHEAD)) << (2 - BIO_RW_AHEAD))
> include/linux/bio.h:#define BIO_RW_AHEAD        1
> include/linux/bio.h:#define bio_rw_ahead(bio)   ((bio)->bi_rw & (1 << BIO_RW_AHEAD))
> willy@...eydew:~/kernel/linux-2.6$ grep -r bio_rw_ahead *
> block/ll_rw_blk.c:      if (bio_rw_ahead(bio) || bio_failfast(bio))
> drivers/md/dm-mpath.c:  if ((error == -EWOULDBLOCK) && bio_rw_ahead(bio))
> drivers/md/multipath.c: else if (!bio_rw_ahead(bio)) {
> include/linux/bio.h:#define bio_rw_ahead(bio)   ((bio)->bi_rw & (1 << BIO_RW_AHEAD))

That would say to me that READA is not hooked up correctly. i.e:

#define READ 0
#define WRITE 1
#define READA 2         /* read-ahead  - don't block if no resources */
#define SWRITE 3        /* for ll_rw_block() - wait for buffer lock */
#define READ_SYNC       (READ | (1 << BIO_RW_SYNC))
#define READ_META       (READ | (1 << BIO_RW_META))
#define WRITE_SYNC      (WRITE | (1 << BIO_RW_SYNC))
#define WRITE_BARRIER   ((1 << BIO_RW) | (1 << BIO_RW_BARRIER))

i.e. it should be:

#define READA		(1 << BIO_RW_AHEAD)

Right?

FWIW, dm does this:

		if (bio_rw(bio) != READA)

Which really should be if (bio_rw_ahead(bio)).....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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