Re: [PATCH] block: fix nr_phys_segments miscalculation bug

[Jens Axboe <jens.axboe@oracle.com>]

On Sat, Oct 11 2008, FUJITA Tomonori wrote:
> This is against the latest git (b922df7383749a1c0b7ea64c50fa839263d3816b).
> 
> =
> From: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
> Subject: [PATCH] block: fix nr_phys_segments miscalculation bug
> 
> This fixes the bug reported by Nikanth Karthikesan <knikanth@suse.de>:
> 
> http://lkml.org/lkml/2008/10/2/203
> 
> The root cause of the bug is that blk_phys_contig_segment
> miscalculates q->max_segment_size.
> 
> blk_phys_contig_segment checks:
> 
> req->biotail->bi_size + next_req->bio->bi_size > q->max_segment_size
> 
> But blk_recalc_rq_segments might expect that req->biotail and the
> previous bio in the req are supposed be merged into one
> segment. blk_recalc_rq_segments might also expect that next_req->bio
> and the next bio in the next_req are supposed be merged into one
> segment. In such case, we merge two requests that can't be merged
> here. Later, blk_rq_map_sg gives more segments than it should.
> 
> We need to keep track of segment size in blk_recalc_rq_segments and
> use it to see if two requests can be merged. This patch implements it
> in the similar way that we used to do for hw merging (virtual
> merging).

This looks really good, just like I imagined. I'll give it a fuller
review later today and do a bit of targetted testing, if it goes as
planned it'll go in soonish. Thanks a lot!

-- 
Jens Axboe