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SubjectRe: BLKSECDISCARD ioctl and hung tasks
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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.

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