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Date: Sun, 12 Jan 2014 19:52:40 -0800 (PST) From: Hugh Dickins <hughd@...gle.com> To: Jens Axboe <axboe@...nel.dk> cc: Kent Overstreet <kmo@...erainc.com>, Shaohua Li <shli@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org Subject: next bio iters break discard? 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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