On 11/20/20 1:52 AM, Christoph Hellwig wrote:
On Tue, Nov 17, 2020 at 03:56:24PM +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>
---
block/blk-merge.c | 7 +++++++
block/blk-mq.c | 6 ++++--
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index bcf5e4580603..53ad781917a2 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -279,6 +279,13 @@ 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 iopoll,
Please capitalize the first character of a multi-line comments. Also
this adds an overly long line.
Regards.
+ hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
+ if (hctx->type != HCTX_TYPE_POLL)
+ return 0;
I think this is good as a sanity check, but shouldn't we be able to
avoid even hitting this patch if we ensure that BLK_QC_T_NONE is
returned after a bio is split?
Actually I had thought about returning BLK_QC_T_NONE for split bio, but
got blocked.
At the beginning, I want to identify split bio by checking if @split is
NULL in __blk_queue_split().
```
split = blk_bio_segment_split(q, *bio, &q->bio_split,
nr_segs);
break;
}
if (split) {
/* bio got split */
```
But it's not the case. Even if @split is NULL, the input @bio may be the
*last* split bio.
Then I want to identify split bio by checking loop times in
__submit_bio_noacct_mq().
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1008,12 +1008,15 @@ static blk_qc_t __submit_bio_noacct_mq(struct
bio *bio)
{
struct bio_list bio_list[2] = { };
blk_qc_t ret = BLK_QC_T_NONE;
+ int split = -1;
current->bio_list = bio_list;
do {
struct gendisk *disk = bio->bi_disk;
+ split = min(split + 1, 1)
+
if (unlikely(bio_queue_enter(bio) != 0))
continue;
@@ -1027,7 +1030,7 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio
*bio)
} while ((bio = bio_list_pop(&bio_list[0])));
current->bio_list = NULL;
- return ret;
+ return split ? BLK_QC_T_NONE : ret;
}
But the bio-based routine will call blk_mq_submit_bio() directly, bypassing
__submit_bio_noacct_mq().
It seems that we have to add one specific flag to identify split bio.
Or we could use BIO_CHAIN to identify the *last* split bio from normal
bio, since the
last split bio is always marked with BIO_CHAIN. Then we can identify the
last split
bio by BIO_CHAIN, and the others by checking if @split is NULL in
__blk_queue_split().
--
Thanks,
Jeffle
