IOpoll is initially for small size, latency sensitive IO. It doesn't work well for big IO, especially when it needs to be split to multiple bios. When it comes to bio split, the returned cookie of __submit_bio_noacct_mq() is indeed the cookie of the last split bio. The completion of *this* last split bio done by iopoll doesn't mean the whole original bio has completed. Callers of iopoll still need to wait for completion of other split bios. Besides bio splitting may cause more trouble for iopoll which isn't supposed to be used in case of big IO. IOpoll for split bio may cause potential race if CPU migration happens during bio submission. Since the returned cookie is that of the last split bio of, polling on the corresponding hardware queue doesn't help complete other split bios, if these split bios are enqueued into different hardware queues. Since interrupts are disabled for polling queues, the completion of these other split bios depends on timeout mechanism, thus causing a potential IO hang. IOpoll for split bio may also cause hang for sync polling. Currently both the blkdev and iomap-based fs (ext4/xfs, etc) support sync polling in direct IO routine. These routines will submit bio without REQ_NOWAIT flag set, and then start sync polling in current process context. The process may hang in blk_mq_get_tag() if the submitted bio has to be split into multiple bios and can rapidly exhaust the queue depth. The process are waiting for the completion of the previously allocated requests, which should be done by the following polling, and thus causing a deadlock. To avoid these subtle trouble described above, just disable iopoll for split bio. Suggested-by: Ming Lei <ming.lei@xxxxxxxxxx> Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> --- block/blk-merge.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/block/blk-merge.c b/block/blk-merge.c index bcf5e4580603..dafd5ec65545 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -279,6 +279,22 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, return NULL; split: *segs = nsegs; + + /* + * IOpoll is initially for small size, latency sensitive IO. + * + * It doesn't work well for big IO, especially when it needs to be split to + * multiple bios. When it comes to bio split, the returned cookie of + * __submit_bio_noacct_mq() is indeed the cookie of the last split bio. The + * completion of *this* last split bio done by polling doesn't mean the whole + * original bio has completed. Callers of polling still need to wait for + * completion of other split bios. + * + * Besides bio splitting may cause more trouble for iopoll which isn't supposed + * to be used in case of big IO. + */ + bio->bi_opf &= ~REQ_HIPRI; + return bio_split(bio, sectors, GFP_NOIO, bs); } -- 2.27.0
