On Wed, Nov 25, 2020 at 04:05:10PM +0800, JeffleXu wrote: > > > On 11/25/20 3:19 PM, Ming Lei wrote: > > 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); > >> + > >> return bio_split(bio, sectors, GFP_NOIO, bs); > >> } > >> > >> diff --git a/block/blk-mq.c b/block/blk-mq.c > >> index 55bcee5dc032..ce1f3628e4c2 100644 > >> --- a/block/blk-mq.c > >> +++ b/block/blk-mq.c > >> @@ -2265,6 +2265,9 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio) > >> blk_mq_sched_insert_request(rq, false, true, true); > >> } > >> > >> + if (bio_flagged(bio, BIO_SPLIT)) > >> + return BLK_QC_T_NONE; > >> + > > > > Not sure the new bio flag is really required for this case, just wondering > > why not take the following simple way? BTW we are really going to run > > out of bio flag. > > > > Please consider the following case: > > One big bio got split into two split bios. At the first call of > blk_mq_submit_bio(), the input @bio (actually the original big bio) > indeed gets split. The split bio gets enqueued to hw queue and the > returned cookie is BLK_QC_T_NONE, while the remained bio gets buffered > in bio_list. So far so good. When this bio gets splitted, REQ_HIPRI is cleared for this bio, and all splitted bios won't set this flag too. > > Then when calling blk_mq_submit_bio() the second time, the input @bio is > indeed the remained bio. At this time, it will not get split and you > will get a *valid* cookie. And since the cookie of last split bio will > actually overrides the previous cookie, you will get a *valid* cookie as > a result. Then valid cookie can be returned only for bio with REQ_HIPRI. Thanks, Ming
