On Wed, Nov 25, 2020 at 02:41:47PM +0800, Jeffle Xu wrote: > 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. In this case, 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, 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 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 reaped 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> > Reviewed-by: Christoph Hellwig <hch@xxxxxx> > --- > block/bio.c | 2 ++ > block/blk-merge.c | 12 ++++++++++++ > block/blk-mq.c | 3 +++ > include/linux/blk_types.h | 1 + > 4 files changed, 18 insertions(+) > > diff --git a/block/bio.c b/block/bio.c > index fa01bef35bb1..7f7ddc22a30d 100644 > --- a/block/bio.c > +++ b/block/bio.c > @@ -684,6 +684,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src) > bio_set_flag(bio, BIO_CLONED); > if (bio_flagged(bio_src, BIO_THROTTLED)) > bio_set_flag(bio, BIO_THROTTLED); > + if (bio_flagged(bio_src, BIO_SPLIT)) > + bio_set_flag(bio, BIO_SPLIT); > bio->bi_opf = bio_src->bi_opf; > bio->bi_ioprio = bio_src->bi_ioprio; > bio->bi_write_hint = bio_src->bi_write_hint; > diff --git a/block/blk-merge.c b/block/blk-merge.c > index bcf5e4580603..a2890cebf99f 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -279,6 +279,18 @@ static struct bio *blk_bio_segment_split(struct request_queue *q, > return NULL; > split: > *segs = nsegs; > + > + /* > + * Bio splitting may cause subtle trouble such as hang when doing sync > + * iopoll in direct IO routine. Given performance gain of iopoll for > + * big IO can be trival, disable iopoll when split needed. We need > + * BIO_SPLIT to identify bios need this workaround. Since currently > + * only normal IO under mq routine may suffer this issue, BIO_SPLIT is > + * only marked here. > + */ > + bio->bi_opf &= ~REQ_HIPRI; > + bio_set_flag(bio, BIO_SPLIT); You may need to put the above into one helper, and call the helper for other splitted cases(discard, write zero and write same) too. thanks, Ming
