From: Salman Qazi <sqazi@google.com>
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>
Subject: Re: BLKSECDISCARD ioctl and hung tasks
Date: Thu, 13 Feb 2020 11:21:37 -0800 [thread overview]
Message-ID: <CAKUOC8U1H8qJ+95pcF-fjeu9hag3P3Wm6XiOh26uXOkvpNngZg@mail.gmail.com> (raw)
In-Reply-To: <d2c77921-fdcd-4667-d21a-60700e6a2fa5@acm.org>
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.
next prev parent reply other threads:[~2020-02-13 19:21 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 22:27 BLKSECDISCARD ioctl and hung tasks Salman Qazi
2020-02-12 23:06 ` Theodore Y. Ts'o
2020-02-13 1:20 ` Salman Qazi
2020-02-13 1:24 ` Jesse Barnes
2020-02-13 8:26 ` Ming Lei
2020-02-13 17:48 ` Bart Van Assche
2020-02-13 19:21 ` Salman Qazi [this message]
2020-02-13 22:08 ` Salman Qazi
2020-02-14 0:25 ` Ming Lei
2020-02-14 5:49 ` Bart Van Assche
2020-02-14 9:22 ` Ming Lei
2020-02-14 19:42 ` Salman Qazi
2020-02-15 3:46 ` Ming Lei
2020-02-18 16:11 ` Jesse Barnes
2020-02-19 1:37 ` Ming Lei
2020-02-19 2:54 ` Ming Lei
2020-02-19 17:54 ` Salman Qazi
2020-02-19 22:22 ` Ming Lei
2020-02-19 22:26 ` Salman Qazi
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