blk-mq: fix for flush deadlock

The flush state machine takes in a struct request, which then is
submitted multiple times to the underling driver.  The old block code
requeses the same request for each of those, so it does not have an
issue with tapping into the request pool.  The new one on the other hand
allocates a new request for each of the actualy steps of the flush
sequence. If have already allocated all of the tags for IO, we will
fail allocating the flush request.

Set aside a reserved request just for flushes.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 1 insertions, 1 deletions

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 3e4cc9c7890a..331e627301ea 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -286,7 +286,7 @@ static void mq_flush_work(struct work_struct *work)
 
         /* We don't need set REQ_FLUSH_SEQ, it's for consistency */
         rq = blk_mq_alloc_request(q, WRITE_FLUSH|REQ_FLUSH_SEQ,
-                __GFP_WAIT|GFP_ATOMIC);
+                __GFP_WAIT|GFP_ATOMIC, true);
         rq->cmd_type = REQ_TYPE_FS;
         rq->end_io = flush_end_io;