On Thu, Feb 13, 2020 at 9:48 AM Bart Van Assche [off-list ref] wrote:

On 2/13/20 12:26 AM, Ming Lei wrote:

quoted

The approach used in blk_execute_rq() can be borrowed for workaround the
issue, such as:

diff --git a/block/bio.c b/block/bio.c
index 94d697217887..c9ce19a86de7 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -17,6 +17,7 @@
  #include <linux/cgroup.h>
  #include <linux/blk-cgroup.h>
  #include <linux/highmem.h>
+#include <linux/sched/sysctl.h>

  #include <trace/events/block.h>
  #include "blk.h"

@@ -1019,12 +1020,19 @@ static void submit_bio_wait_endio(struct bio *bio)
  int submit_bio_wait(struct bio *bio)
  {
      DECLARE_COMPLETION_ONSTACK_MAP(done, bio->bi_disk->lockdep_map);
+     unsigned long hang_check;

      bio->bi_private = &done;
      bio->bi_end_io = submit_bio_wait_endio;
      bio->bi_opf |= REQ_SYNC;
      submit_bio(bio);
-     wait_for_completion_io(&done);
+
+     /* Prevent hang_check timer from firing at us during very long I/O */
+     hang_check = sysctl_hung_task_timeout_secs;
+     if (hang_check)
+             while (!wait_for_completion_io_timeout(&done, hang_check * (HZ/2)));
+     else
+             wait_for_completion_io(&done);

      return blk_status_to_errno(bio->bi_status);
  }

Instead of suppressing the hung task complaints, has it been considered
to use the bio splitting mechanism to make discard bios smaller? Block
drivers may set a limit by calling blk_queue_max_discard_segments().
 From block/blk-settings.c:

/**
  * blk_queue_max_discard_segments - set max segments for discard
  * requests
  * @q:  the request queue for the device
  * @max_segments:  max number of segments
  *
  * Description:
  *    Enables a low level driver to set an upper limit on the number of
  *    segments in a discard request.
  **/
void blk_queue_max_discard_segments(struct request_queue *q,
                unsigned short max_segments)
{
        q->limits.max_discard_segments = max_segments;
}
EXPORT_SYMBOL_GPL(blk_queue_max_discard_segments);

AFAICT, This is not actually sufficient, because the issuer of the bio
is waiting for the entire bio, regardless of how it is split later.
But, also there isn't a good mapping between the size of the secure
discard and how long it will take.  If given the geometry of a flash
device, it is not hard to construct a scenario where a relatively
small secure discard (few thousand sectors) will take a very long time
(multiple seconds).

Having said that, I don't like neutering the hung task timer either.

Thanks,

Bart.