Re: ext2 on a CD-RW

[Peter Osterlund <petero2@telia.com>]

Jens Axboe <axboe@suse.de> writes:

> On Fri, Jan 02 2004, Peter Osterlund wrote:
> > 
> > Old versions of the packet writing code did just that, but Jens told
> > me that bio splitting was evil, so when the merge_bvec_fn
> > functionality was added to the kernel, I started to use it.
> > 
> >         http://lists.suse.com/archive/packet-writing/2002-Aug/0044.html
> 
> Splitting is evil, but unfortunately it's a necessary evil... There are
> a few kernel helpers to make supporting it easier (see bio_split()). Not
> so sure how well that'll work for you, you may have to do the grunt work
> yourself.

It seems like bio_split() does exactly what I need. The patch below
makes 2kb blocksize ext2 work and also fixes the performance problem
compared to 4kb blocksize ext2. Thanks to everyone involved for their
help.

--- linux/drivers/block/pktcdvd.c.old	2004-01-02 16:58:52.000000000 +0100
+++ linux/drivers/block/pktcdvd.c	2004-01-02 16:59:26.000000000 +0100
@@ -2083,11 +2083,23 @@
 		(unsigned long long)bio->bi_sector,
 		(unsigned long long)(bio->bi_sector + bio_sectors(bio)));
 
-	/* Some debug code to make sure the merge_bvec_fn function is working. */
+	/* Check if we have to split the bio */
 	{
+		struct bio_pair *bp;
 		sector_t last_zone;
+		int first_sectors;
+
 		last_zone = ZONE(bio->bi_sector + bio_sectors(bio) - 1, pd);
-		BUG_ON(last_zone != zone);
+		if (last_zone != zone) {
+			BUG_ON(last_zone != zone + pd->settings.size);
+			first_sectors = last_zone - bio->bi_sector;
+			bp = bio_split(bio, bio_split_pool, first_sectors);
+			BUG_ON(!bp);
+			pkt_make_request(q, &bp->bio1);
+			pkt_make_request(q, &bp->bio2);
+			bio_pair_release(bp);
+			return 0;
+		}
 	}
 
 	/*
@@ -2153,6 +2165,15 @@
 	sector_t zone = ZONE(bio->bi_sector, pd);
 	int used = ((bio->bi_sector - zone) << 9) + bio->bi_size;
 	int remaining = (pd->settings.size << 9) - used;
+	int remaining2;
+
+	/*
+	 * A bio <= PAGE_SIZE must be allowed. If it crosses a packet
+	 * boundary, pkt_make_request() will split the bio.
+	 */
+	remaining2 = PAGE_SIZE - bio->bi_size;
+	remaining = max_t(int, remaining, remaining2);
+
 	BUG_ON(remaining < 0);
 	return remaining;
 }

-- 
Peter Osterlund - petero2@telia.com
http://w1.894.telia.com/~u89404340