quoted

quoted

I'm bisecting now, there are a couple of patches from Ming in
the area of the bio splitting code...

CC'ing Ming, Linux-block and Linux-nvme as iser is identical to nvme
wrt the virtual boundary so I think nvme will break as well.

Bisection reveals that this one is the culprit:

commit 52cc6eead9095e2faf2ec7afc013aa3af1f01ac5
Author: Ming Lei [off-list ref]
Date:   Thu Sep 17 09:58:38 2015 -0600

    block: blk-merge: fast-clone bio when splitting rw bios

    biovecs has become immutable since v3.13, so it isn't necessary
    to allocate biovecs for the new cloned bios, then we can save
    one extra biovecs allocation/copy, and the allocation is often
    not fixed-length and a bit more expensive.

    For example, if the 'max_sectors_kb' of null blk's queue is set
    as 16(32 sectors) via sysfs just for making more splits, this patch
    can increase throught about ~70% in the sequential read test over
    null_blk(direct io, bs: 1M).

    Cc: Christoph Hellwig [off-list ref]
    Cc: Kent Overstreet [off-list ref]
    Cc: Ming Lin [off-list ref]
    Cc: Dongsu Park [off-list ref]
    Signed-off-by: Ming Lei [off-list ref]

    This fixes a performance regression introduced by commit 54efd50bfd,
    and allows us to take full advantage of the fact that we have
immutable
    bio_vecs. Hand applied, as it rejected violently with commit
    5014c311baa2.

    Signed-off-by: Jens Axboe [off-list ref]
--

Looks like there is a problem with bio_clone_fast()

This change makes the problem go away:
--

diff --git a/block/bio.c b/block/bio.c
index dbabd48..5e93733 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -1791,7 +1791,7 @@ struct bio *bio_split(struct bio *bio, int sectors,
         * Discards need a mutable bio_vec to accommodate the payload
         * required by the DSM TRIM and UNMAP commands.
         */
-       if (bio->bi_rw & REQ_DISCARD)
+       if (1 || bio->bi_rw & REQ_DISCARD)
                split = bio_clone_bioset(bio, gfp, bs);
        else
                split = bio_clone_fast(bio, gfp, bs);
--

Any thoughts?