Setting the dm-crypt device's max_segment_size to PAGE_SIZE is an
unfortunate constraint that is required to avoid the potential for
exceeding dm-crypt's underlying device's max_segments limits -- due to
crypt_alloc_buffer() possibly allocating pages for the encryption bio
that are not as physically contiguous as the original bio.

It is interesting to note that this problem was already fixed back in
2007 via commit 91e10625 ("dm crypt: use bio_add_page").  But Linux 4.0
commit cf2f1abf ("dm crypt: don't allocate pages for a partial
request") regressed dm-crypt back to _not_ using bio_add_page().  But
given dm-crypt's cpu parallelization changes all depend on commit
cf2f1abf's abandoning of the more complex io fragments processing that
dm-crypt previously had we cannot easily go back to using
bio_add_page().

So all said the cleanest way to resolve this issue is to fix dm-crypt to
properly constrain the original bios entering dm-crypt so the encryption
bios that dm-crypt generates from the original bios are always
compatible with the underlying device's max_segments queue limits.

It should be noted that technically Linux 4.3 does _not_ need this fix
because of the block core's new late bio-splitting capability.  But, it
is reasoned, there is little to be gained by having the block core split
the encrypted bio that is composed of PAGE_SIZE segments.  That said, in
the future we may revert this change.

Fixes: cf2f1abf ("dm crypt: don't allocate pages for a partial request")
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=104421Suggested-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.0+