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;