Re: e827091cb1 "block: merge: get the 1st and last bvec via helpers" broken

[Kent Overstreet]

Here's the output of the patch below:

generic/036 11s ...run fstests generic/036 at 2016-03-12 13:58:21
end 4096 0 ffffea0001d611c0 end2 1024 0 ffffea0001d611c0
len 1024 offset 0 page ffffea0001d611c0
KGDB: Waiting for remote debugger

Your code gives a biovec with bv_len of 4096, the old code gives a biovec with
bv_len of 1024 (and then we dump every biovec, we see that the bio had only a
single biovec that did indeed have bv_len == 1024).

bio_get_last_bvec() is broken, please revert your patch for v4.5.

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 261353166d..231aec4fa3 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -318,6 +318,34 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
 	bio_get_last_bvec(bio, &end_bv);
 	bio_get_first_bvec(nxt, &nxt_bv);
 
+	{
+		struct bvec_iter iter;
+		struct bio_vec end2, bv;
+
+		bio_for_each_segment(end2, bio, iter)
+			if (end2.bv_len == iter.bi_size)
+				break;
+
+		if (end_bv.bv_len	!= end2.bv_len ||
+		    end_bv.bv_offset	!= end2.bv_offset ||
+		    end_bv.bv_page	!= end2.bv_page) {
+			printk(KERN_ERR "end %u %u %p end2 %u %u %p\n",
+			      end_bv.bv_len,
+			      end_bv.bv_offset,
+			      end_bv.bv_page,
+			      end2.bv_len,
+			      end2.bv_offset,
+			      end2.bv_page);
+
+			bio_for_each_segment(bv, bio, iter)
+				printk(KERN_ERR "len %u offset %u page %p\n",
+				       bv.bv_len,
+				       bv.bv_offset,
+				       bv.bv_page);
+			BUG();
+		}
+	}
+
 	if (!BIOVEC_PHYS_MERGEABLE(&end_bv, &nxt_bv))
 		return 0;