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SubjectRe: BLKSECDISCARD ioctl and hung tasks
On Thu, Feb 13, 2020 at 9:48 AM Bart Van Assche <bvanassche@acm.org> wrote:
>
> 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);
>

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.

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