On Mon, Nov 27 2017, Mike Snitzer wrote:

On Thu, Nov 23 2017 at  5:52pm -0500,
NeilBrown [off-list ref] wrote:

quoted

When we use bio_clone_bioset() to split off the front part of a bio
and chain the two together and submit the remainder to
generic_make_request(), it is important that the newly allocated
bio is used as the head to be processed immediately, and the original
bio gets "bio_advance()"d and sent to generic_make_request() as the
remainder.

If the newly allocated bio is used as the remainder, and if it then
needs to be split again, then the next bio_clone_bioset() call will
be made while holding a reference a bio (result of the first clone)
from the same bioset.  This can potentially exhaust the bioset mempool
and result in a memory allocation deadlock.

So the result of the bio_clone_bioset() must be attached to the new
dm_io struct, and the original must be resubmitted.  The current code
is backwards.

Note that there is no race caused by reassigning cio.io->bio after already
calling __map_bio().  This bio will only be dereferenced again after
dec_pending() has found io->io_count to be zero, and this cannot happen
before the dec_pending() call at the end of __split_and_process_bio().

Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: NeilBrown <redacted>
---

Hi,
 I think this should resolve the problem Mikulas noticed that the
 bios form a deep chain instead of a wide tree.

I'm inclined to just fold this into the original commit.
I'd update that header to make mention of the details captured in this
header.

Would you be OK with that?

Perfectly OK with that.  Thanks for asking.

NeilBrown