Re: [PATCH 5/8] nowait aio: return on congested block device

On Thu, Mar 16, 2017 at 10:33 PM, Jens Axboe [off-list ref] wrote:

On 03/15/2017 03:51 PM, Goldwyn Rodrigues wrote:

quoted

diff --git a/block/blk-core.c b/block/blk-core.c
index 0eeb99e..2e5cba2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -2014,7 +2019,7 @@ blk_qc_t generic_make_request(struct bio *bio)
      do {
              struct request_queue *q = bdev_get_queue(bio->bi_bdev);

-             if (likely(blk_queue_enter(q, false) == 0)) {
+             if (likely(blk_queue_enter(q, bio_flagged(bio, BIO_NOWAIT)) == 0)) {
                      struct bio_list hold;
                      struct bio_list lower, same;

@@ -2040,7 +2045,10 @@ blk_qc_t generic_make_request(struct bio *bio)
                      bio_list_merge(&bio_list_on_stack, &same);
                      bio_list_merge(&bio_list_on_stack, &hold);
              } else {
-                     bio_io_error(bio);
+                     if (unlikely(bio_flagged(bio, BIO_NOWAIT)))
+                             bio_wouldblock_error(bio);
+                     else
+                             bio_io_error(bio);

This doesn't look right. What if the queue is dying, and BIO_NOWAIT just
happened to be set?

And you're missing wbt_wait() as well as a blocking point. Ditto in
blk-mq.

There are also tons of block points in dm/md/bcache, which need to be
considered or be documented at least, :-)



Thanks,
Ming Lei