On 10/16/20 6:26 PM, Ming Lei wrote:

On Fri, Oct 16, 2020 at 05:18:51PM +0800, Jeffle Xu wrote:

quoted

Both blkdev fs and iomap-based fs (ext4, xfs, etc.) currently support
sync iopoll. One single bio can contain at most BIO_MAX_PAGES, i.e. 256
bio_vec. If the input iov_iter contains more than 256 segments, then
one dio will be split into multiple bios, which may cause potential
deadlock for sync iopoll.

When it comes to sync iopoll, the bio is submitted without REQ_NOWAIT
flag set and the process may hang in blk_mq_get_tag() if the dio needs
to be split into multiple bios and thus can rapidly exhausts the queue
depth. The process has to wait for the completion of the previously
allocated requests, which should be reaped by the following sync
polling, and thus causing a deadlock.

In fact there's a subtle difference of handling of HIPRI IO between
blkdev fs and iomap-based fs, when dio need to be split into multiple
bios. blkdev fs will set REQ_HIPRI for only the last split bio, leaving
the previous bios queued into normal hardware queues, and not causing
the trouble described above. iomap-based fs will set REQ_HIPRI for all
split bios, and thus may cause the potential deadlock decribed above.

Thus disable iopoll when one dio need to be split into multiple bios.
Though blkdev fs may not suffer this issue, still it may not make much
sense to iopoll for big IO, since iopoll is initially for small size,
latency sensitive IO.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
  fs/block_dev.c       | 7 +++++++
  fs/iomap/direct-io.c | 8 ++++++++
  2 files changed, 15 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9e84b1928b94..1b56b39e35b5 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c

@@ -436,6 +436,13 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
  			break;
  		}
  
+		/*
+		 * The current dio need to be split into multiple bios here.
+		 * iopoll is initially for small size, latency sensitive IO,
+		 * and thus disable iopoll if split needed.
+		 */
+		iocb->ki_flags &= ~IOCB_HIPRI;
+

Not sure if it is good to clear IOCB_HIPRI of iocb, since it is usually
maintained by upper layer code(io_uring, aio, ...) and we shouldn't
touch this flag here.

If we queue bios into the DEFAULT hardware queue, but leaving the 
corresponding kiocb->ki_flags's

IOCB_HIPRI set (exactly what the first patch does), is this another
inconsistency?

Please consider the following code snippet from __blkdev_direct_IO()

	for (;;) {
		set_current_state(TASK_UNINTERRUPTIBLE);
		if (!READ_ONCE(dio->waiter))
			break;

		if (!(iocb->ki_flags & IOCB_HIPRI) ||
		    !blk_poll(bdev_get_queue(bdev), qc, true))
			blk_io_schedule();
	}

The IOCB_HIPRI flag is still set in iocb->ki_flags, but the corresponding
bios are queued into DEFAULT hardware queue since the first patch.
blk_poll() is still called in this case.

quoted

  		if (!dio->multi_bio) {
  			/*
  			 * AIO needs an extra reference to ensure the dio

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c1aafb2ab990..46668cceefd2 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c

@@ -308,6 +308,14 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
  		copied += n;
  
  		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
+		/*
+		 * The current dio need to be split into multiple bios here.
+		 * iopoll is initially for small size, latency sensitive IO,
+		 * and thus disable iopoll if split needed.
+		 */
+		if (nr_pages)
+			dio->iocb->ki_flags &= ~IOCB_HIPRI;

Same concern as above.

Thanks,
Ming