block: Improvements to bounce-buffer handling
authorNeilBrown <neilb@suse.com>
Sun, 18 Jun 2017 04:38:58 +0000 (14:38 +1000)
committerJens Axboe <axboe@kernel.dk>
Sun, 18 Jun 2017 18:40:59 +0000 (12:40 -0600)
commita8821f3f32bea173c7f9546b3b025898f0e09f58
treed911354d3c07a5663256d5684d87f8d27435bdb6
parent93b27e72904a9869e648c870bf0d04b124fda1c7
block: Improvements to bounce-buffer handling

Since commit 23688bf4f830 ("block: ensure to split after potentially
bouncing a bio") blk_queue_bounce() is called *before*
blk_queue_split().
This means that:
 1/ the comments blk_queue_split() about bounce buffers are
    irrelevant, and
 2/ a very large bio (more than BIO_MAX_PAGES) will no longer be
    split before it arrives at blk_queue_bounce(), leading to the
    possibility that bio_clone_bioset() will fail and a NULL
    will be dereferenced.

Separately, blk_queue_bounce() shouldn't use fs_bio_set as the bio
being copied could be from the same set, and this could lead to a
deadlock.

So:
 - allocate 2 private biosets for blk_queue_bounce, one for
   splitting enormous bios and one for cloning bios.
 - add code to split a bio that exceeds BIO_MAX_PAGES.
 - Fix up the comments in blk_queue_split()

Credit-to: Ming Lei <tom.leiming@gmail.com> (suggested using single bio_for_each_segment loop)
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block/blk-merge.c
block/bounce.c