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SubjectRe: [PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly
On Tue, 23 Sep 2008 23:58:02 +0530
Nikanth Karthikesan <> wrote:

> From: Nikanth Karthikesan <>
> ll_merge_requests_fn() decreases the req->nr_phys_segments if
> blk_phys_contig_segment() returns true, but it is perfectly possible that
> blk_hw_contig_segment() is false.

Yeah, in fact, blk_hw_contig_segment() is always false on the majority
of architectures (on only PARISC and Alpha, it could be true).

Your patch doesn't look correct. Virtually, the patch always disables
physical merging.

> A new hw_segment implies a new phys_segment.
> So decrementing nr_phys_segments wrongly here triggers the BUG_ON() in
> scsi_init_sgtable(), as blk_rq_map_sg() would map 1 phys_segment more than
> req->nr_phys_segment. This is easily reproducible. Other callers of
> blk_rq_map_sg() should also be affected by this bug.

I have no idea how BUG_ON() in scsi_init_sgtable() is triggered.

Can you give more information, HBA, IOMMU (if you use), and the values
of req->nr_phys_segment, req->nr_hw_segment, count, etc in in
scsi_init_sgtable() when you hit the bug?

BTW, blk_hw_contig_segment() will be removed for 2.6.28 (virtual
merging account in the block layer will be removed).

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