Re: [PATCH v3 2/2] block,iomap: disable iopoll when split needed

[Ming Lei <ming.lei@redhat.com>]

On Fri, Oct 16, 2020 at 07:02:44PM +0800, JeffleXu wrote:
> 
> On 10/16/20 6:26 PM, Ming Lei wrote:
> > On Fri, Oct 16, 2020 at 05:18:51PM +0800, Jeffle Xu wrote:
> > > 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?

My question is that if it is good for this code to clear IOCB_HIPRI of iocb,
given this is the 1st such usage. And does io_uring implementation expect
the flag to be cleared by lower layer?

> 
> 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.

It may be handled in the following way:

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

BTW, even for single bio with IOCB_HIPRI, the single fs bio can still be
splitted, and blk_poll() will be called too.


Thanks, 
Ming