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From:   Salman Qazi <sqazi@google.com>
Date:   Thu, 13 Feb 2020 11:21:37 -0800
Message-ID: <CAKUOC8U1H8qJ+95pcF-fjeu9hag3P3Wm6XiOh26uXOkvpNngZg@mail.gmail.com>
Subject: Re: BLKSECDISCARD ioctl and hung tasks
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Ming Lei <ming.lei@redhat.com>, 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>
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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.