next bio iters break discard?

[Hugh Dickins]

When I try to exercise heavy swapping with discard on mmotm 2014-01-09,
I soon hit a NULL pointer dereference in __blk_recalc_rq_segments():

__blk_recalc_rq_segments
blk_recount_segments
ll_back_merge_fn
bio_attempt_back_merge
blk_queue_bio
generic_make_request
submit_bio
blkdev_issue_discard
swap_do_scheduled_discard
scan_swap_map_try_ssd_cluster
scan_swap_map
get_swap_page
add_to_swap
shrink_page_list
etc. etc.

The crash is on the NULL struct page pointer in page_to_pfn(bv.bv_page)
on line 35 of block/blk-merge.c.

The code around there is not very different from 3.13-rc8 (which doesn't
crash), and I didn't notice REQ_DISCARD or bio_has_data() checks removed.

I think it worked before because the old bio_for_each_segment()
iterator was a straightforward "i < bio->bi_vcnt" loop which would
do nothing when bi_vcnt is 0; but the new iterators are relying
(perhaps) on bio->bi_iter.bi_size which is non-0 despite no data?

I expect it would crash in the same way on other recent nexts and
mmotms, I've not tried.

Hugh
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