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Subject: Re: BLKSECDISCARD ioctl and hung tasks
To:     Ming Lei <ming.lei@redhat.com>, Salman Qazi <sqazi@google.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-block@vger.kernel.org, Gwendal Grignou <gwendal@google.com>,
        Jesse Barnes <jsbarnes@google.com>
References: <CAKUOC8VN5n+YnFLPbQWa1hKp+vOWH26FKS92R+h4EvS=e11jFA@mail.gmail.com>
 <20200213082643.GB9144@ming.t460p>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <d2c77921-fdcd-4667-d21a-60700e6a2fa5@acm.org>
Date:   Thu, 13 Feb 2020 09:48:25 -0800
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On 2/13/20 12:26 AM, Ming Lei wrote:
> 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);

Thanks,

Bart.