On 10/15/20 3:59 PM, Christoph Hellwig wrote:
On Thu, Oct 15, 2020 at 03:40:31PM +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
the IO request described by this iov_iter 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 input
iov_iter has to be split into multiple bios and thus rapidly exhausts
the queue depth. The process has to wait for the completion of the
previously allocated requests, which should be done by the following
sync polling, and thus causing a deadlock.
Actually there's subtle difference between the behaviour of handling
HIPRI IO of blkdev and iomap, when the input iov_iter need to split
into multiple bios. blkdev will set REQ_HIPRI for only the last split
bio, leaving the previous bio queued into normal hardware queues, which
will not cause the trouble described above though. iomap will set
REQ_HIPRI for all bios split from one iov_iter, and thus may cause the
potential deadlock decribed above.
Disable iopoll when one request need to be split into multiple bios.
Though blkdev may not suffer the problem, 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@xxxxxxxxxxxxxxxxx>
---
fs/block_dev.c | 7 +++++++
fs/iomap/direct-io.c | 9 ++++++++-
2 files changed, 15 insertions(+), 1 deletion(-)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9e84b1928b94..a8a52cab15ab 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -491,6 +491,13 @@ blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
+ /*
+ * IOpoll is initially for small size, latency sensitive IO.
+ * Disable iopoll if split needed.
+ */
+ if (nr_pages > BIO_MAX_PAGES)
+ iocb->ki_flags &= ~IOCB_HIPRI;
more pages than BIO_MAX_PAGES don't imply a split because we can
physically merge pages into a single vector (yes, BIO_MAX_PAGES is
utterly misnamed now).
Sorry I missed it, though the flow may be sometimes misleading -.-||
