Re: [dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

Re: [dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Mike Snitzer]

On Sun, Apr 03 2022 at  7:45P -0400,
Ming Lei <ming.lei@redhat.com> wrote:

> -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is
> set in io_complete_rw_iopoll(), but the req never gets chance to be handled.
> io_iopoll_check doesn't handle this situation, and io hang can be caused.
> 
> Current dm io polling may return -EAGAIN after bio submission is
> returned, also blk-throttle might trigger this situation too.
> 
> Cc: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

I first reverted commit 5291984004ed ("dm: fix bio polling to handle
possibile BLK_STS_AGAIN") then applied this patch and verified this
fixes the DM bio polling hangs.  Nice work!

But interestingly with this fio test (against dm-linear ontop of
null_blk with queue_mode=2 submit_queues=8 poll_queues=2 bs=4096 gb=16):

fio --bs=4096 --ioengine=io_uring --fixedbufs --registerfiles --hipri=1 \
--iodepth=16 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 \
--filename=/dev/mapper/linear --direct=1 --runtime=20 --numjobs=16 \
--rw=randread --name=test --group_reporting --norandommap

I get 3186k IOPS with your patch to have io_uring retry (and commit
5291984004ed reverted), but 4305k IOPS if leave commit 5291984004ed
applied (and DM resorts to retrying any -EAGAIN _without_ polling).

Jens rightly pointed out to me that polling tests that exhaust tags
are bogus anyway (because such unbounded IO defeats the point of
polling).  Jens also thinks my result, with commit 5291984004ed
applied, is somehow bogus and not to be trusted ;)  He is very likely
correct, and the failing likely in the null_blk driver -- I'm
skeptical of that driver given it cannot pass fio verify testing
(e.g. --do_verify=1 --verify=crc32c --verify_async=1) with or without
polling.

Review comments inlined below.

> ---
>  fs/io-wq.h    |  13 +++++
>  fs/io_uring.c | 128 ++++++++++++++++++++++++++++----------------------
>  2 files changed, 86 insertions(+), 55 deletions(-)
> 
> diff --git a/fs/io-wq.h b/fs/io-wq.h
> index dbecd27656c7..4ca4863664fb 100644
> --- a/fs/io-wq.h
> +++ b/fs/io-wq.h
> @@ -96,6 +96,19 @@ static inline void wq_list_add_head(struct io_wq_work_node *node,
>  	WRITE_ONCE(list->first, node);
>  }
>  
> +static inline void wq_list_remove(struct io_wq_work_list *list,
> +				  struct io_wq_work_node *prev,
> +				  struct io_wq_work_node *node)
> +{
> +	if (!prev)
> +		WRITE_ONCE(list->first, node->next);
> +	else
> +		prev->next = node->next;
> +
> +	if (node == list->last)
> +		list->last = prev;
> +}
> +
>  static inline void wq_list_cut(struct io_wq_work_list *list,
>  			       struct io_wq_work_node *last,
>  			       struct io_wq_work_node *prev)
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 59e54a6854b7..6db5514e10ca 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2759,6 +2759,65 @@ static inline bool io_run_task_work(void)
>  	return false;
>  }
>  
> +#ifdef CONFIG_BLOCK
> +static bool io_resubmit_prep(struct io_kiocb *req)
> +{
> +	struct io_async_rw *rw = req->async_data;
> +
> +	if (!req_has_async_data(req))
> +		return !io_req_prep_async(req);
> +	iov_iter_restore(&rw->s.iter, &rw->s.iter_state);
> +	return true;
> +}
> +
> +static bool io_rw_should_reissue(struct io_kiocb *req)
> +{
> +	umode_t mode = file_inode(req->file)->i_mode;
> +	struct io_ring_ctx *ctx = req->ctx;
> +
> +	if (!S_ISBLK(mode) && !S_ISREG(mode))
> +		return false;
> +	if ((req->flags & REQ_F_NOWAIT) || (io_wq_current_is_worker() &&
> +	    !(ctx->flags & IORING_SETUP_IOPOLL)))
> +		return false;
> +	/*
> +	 * If ref is dying, we might be running poll reap from the exit work.
> +	 * Don't attempt to reissue from that path, just let it fail with
> +	 * -EAGAIN.
> +	 */
> +	if (percpu_ref_is_dying(&ctx->refs))
> +		return false;
> +	/*
> +	 * Play it safe and assume not safe to re-import and reissue if we're
> +	 * not in the original thread group (or in task context).
> +	 */
> +	if (!same_thread_group(req->task, current) || !in_task())
> +		return false;
> +	return true;
> +}
> +#else
> +static bool io_resubmit_prep(struct io_kiocb *req)
> +{
> +	return false;
> +}
> +static bool io_rw_should_reissue(struct io_kiocb *req)
> +{
> +	return false;
> +}
> +#endif
> +
> +static void do_io_reissue(struct io_kiocb *req, int ret)
> +{
> +	if (req->flags & REQ_F_REISSUE) {
> +		req->flags &= ~REQ_F_REISSUE;
> +		if (io_resubmit_prep(req))
> +			io_req_task_queue_reissue(req);
> +		else
> +			io_req_task_queue_fail(req, ret);
> +	}
> +}

Minor nit but: I'd leave caller to check for REQ_F_REISSUE.

> +
> +
>  static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
>  {
>  	struct io_wq_work_node *pos, *start, *prev;
> @@ -2786,6 +2845,13 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
>  		if (READ_ONCE(req->iopoll_completed))
>  			break;
>  
> +		/*
> +		 * Once REISSUE flag is set, the req has been done, and we
> +		 * have to retry
> +		 */
> +		if (req->flags & REQ_F_REISSUE)
> +			break;
> +
>  		ret = kiocb->ki_filp->f_op->iopoll(kiocb, &iob, poll_flags);
>  		if (unlikely(ret < 0))
>  			return ret;
> @@ -2807,6 +2873,12 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
>  	wq_list_for_each_resume(pos, prev) {
>  		struct io_kiocb *req = container_of(pos, struct io_kiocb, comp_list);
>  
> +		if (req->flags & REQ_F_REISSUE) {
> +			wq_list_remove(&ctx->iopoll_list, prev, pos);
> +			do_io_reissue(req, -EIO);
> +			break;
> +		}
> +

That way you'll avoid redundant checks for REQ_F_REISSUE here.

Other than that:

Reviewed-by: Mike Snitzer <snitzer@kernel.org>

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Re: [dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Ming Lei]

On Mon, Apr 04, 2022 at 12:51:30PM -0400, Mike Snitzer wrote:
> On Sun, Apr 03 2022 at  7:45P -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is
> > set in io_complete_rw_iopoll(), but the req never gets chance to be handled.
> > io_iopoll_check doesn't handle this situation, and io hang can be caused.
> > 
> > Current dm io polling may return -EAGAIN after bio submission is
> > returned, also blk-throttle might trigger this situation too.
> > 
> > Cc: Mike Snitzer <snitzer@kernel.org>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> I first reverted commit 5291984004ed ("dm: fix bio polling to handle
> possibile BLK_STS_AGAIN") then applied this patch and verified this
> fixes the DM bio polling hangs.  Nice work!
> 
> But interestingly with this fio test (against dm-linear ontop of
> null_blk with queue_mode=2 submit_queues=8 poll_queues=2 bs=4096 gb=16):
> 
> fio --bs=4096 --ioengine=io_uring --fixedbufs --registerfiles --hipri=1 \
> --iodepth=16 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 \
> --filename=/dev/mapper/linear --direct=1 --runtime=20 --numjobs=16 \
> --rw=randread --name=test --group_reporting --norandommap

16jobs in io_uring/aio test is overkill.

> 
> I get 3186k IOPS with your patch to have io_uring retry (and commit
> 5291984004ed reverted), but 4305k IOPS if leave commit 5291984004ed
> applied (and DM resorts to retrying any -EAGAIN _without_ polling).

IMO, commit 5291984004ed shouldn't be reverted, which is reasonable to
retry on underlying IO for dm.

This patch is for making io_uring more reliable, since the current
io_uring code only handles -EAGAIN from submission code path, and
-EAGAIN/REISSUE isn't handled if it is returned during ->poll(),
then the io hang is caused.

Jens, what do you think of this patch? Does io_uring need to handle
-EAGAIN in this case?

> 
> Jens rightly pointed out to me that polling tests that exhaust tags
> are bogus anyway (because such unbounded IO defeats the point of
> polling).  Jens also thinks my result, with commit 5291984004ed
> applied, is somehow bogus and not to be trusted ;)  He is very likely
> correct, and the failing likely in the null_blk driver -- I'm
> skeptical of that driver given it cannot pass fio verify testing
> (e.g. --do_verify=1 --verify=crc32c --verify_async=1) with or without
> polling.

Because it is null block...

> 
> Review comments inlined below.
> 
> > ---
> >  fs/io-wq.h    |  13 +++++
> >  fs/io_uring.c | 128 ++++++++++++++++++++++++++++----------------------
> >  2 files changed, 86 insertions(+), 55 deletions(-)
> > 
> > diff --git a/fs/io-wq.h b/fs/io-wq.h
> > index dbecd27656c7..4ca4863664fb 100644
> > --- a/fs/io-wq.h
> > +++ b/fs/io-wq.h
> > @@ -96,6 +96,19 @@ static inline void wq_list_add_head(struct io_wq_work_node *node,
> >  	WRITE_ONCE(list->first, node);
> >  }
> >  
> > +static inline void wq_list_remove(struct io_wq_work_list *list,
> > +				  struct io_wq_work_node *prev,
> > +				  struct io_wq_work_node *node)
> > +{
> > +	if (!prev)
> > +		WRITE_ONCE(list->first, node->next);
> > +	else
> > +		prev->next = node->next;
> > +
> > +	if (node == list->last)
> > +		list->last = prev;
> > +}
> > +
> >  static inline void wq_list_cut(struct io_wq_work_list *list,
> >  			       struct io_wq_work_node *last,
> >  			       struct io_wq_work_node *prev)
> > diff --git a/fs/io_uring.c b/fs/io_uring.c
> > index 59e54a6854b7..6db5514e10ca 100644
> > --- a/fs/io_uring.c
> > +++ b/fs/io_uring.c
> > @@ -2759,6 +2759,65 @@ static inline bool io_run_task_work(void)
> >  	return false;
> >  }
> >  
> > +#ifdef CONFIG_BLOCK
> > +static bool io_resubmit_prep(struct io_kiocb *req)
> > +{
> > +	struct io_async_rw *rw = req->async_data;
> > +
> > +	if (!req_has_async_data(req))
> > +		return !io_req_prep_async(req);
> > +	iov_iter_restore(&rw->s.iter, &rw->s.iter_state);
> > +	return true;
> > +}
> > +
> > +static bool io_rw_should_reissue(struct io_kiocb *req)
> > +{
> > +	umode_t mode = file_inode(req->file)->i_mode;
> > +	struct io_ring_ctx *ctx = req->ctx;
> > +
> > +	if (!S_ISBLK(mode) && !S_ISREG(mode))
> > +		return false;
> > +	if ((req->flags & REQ_F_NOWAIT) || (io_wq_current_is_worker() &&
> > +	    !(ctx->flags & IORING_SETUP_IOPOLL)))
> > +		return false;
> > +	/*
> > +	 * If ref is dying, we might be running poll reap from the exit work.
> > +	 * Don't attempt to reissue from that path, just let it fail with
> > +	 * -EAGAIN.
> > +	 */
> > +	if (percpu_ref_is_dying(&ctx->refs))
> > +		return false;
> > +	/*
> > +	 * Play it safe and assume not safe to re-import and reissue if we're
> > +	 * not in the original thread group (or in task context).
> > +	 */
> > +	if (!same_thread_group(req->task, current) || !in_task())
> > +		return false;
> > +	return true;
> > +}
> > +#else
> > +static bool io_resubmit_prep(struct io_kiocb *req)
> > +{
> > +	return false;
> > +}
> > +static bool io_rw_should_reissue(struct io_kiocb *req)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> > +static void do_io_reissue(struct io_kiocb *req, int ret)
> > +{
> > +	if (req->flags & REQ_F_REISSUE) {
> > +		req->flags &= ~REQ_F_REISSUE;
> > +		if (io_resubmit_prep(req))
> > +			io_req_task_queue_reissue(req);
> > +		else
> > +			io_req_task_queue_fail(req, ret);
> > +	}
> > +}
> 
> Minor nit but: I'd leave caller to check for REQ_F_REISSUE.
> 
> > +
> > +
> >  static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> >  {
> >  	struct io_wq_work_node *pos, *start, *prev;
> > @@ -2786,6 +2845,13 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> >  		if (READ_ONCE(req->iopoll_completed))
> >  			break;
> >  
> > +		/*
> > +		 * Once REISSUE flag is set, the req has been done, and we
> > +		 * have to retry
> > +		 */
> > +		if (req->flags & REQ_F_REISSUE)
> > +			break;
> > +
> >  		ret = kiocb->ki_filp->f_op->iopoll(kiocb, &iob, poll_flags);
> >  		if (unlikely(ret < 0))
> >  			return ret;
> > @@ -2807,6 +2873,12 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> >  	wq_list_for_each_resume(pos, prev) {
> >  		struct io_kiocb *req = container_of(pos, struct io_kiocb, comp_list);
> >  
> > +		if (req->flags & REQ_F_REISSUE) {
> > +			wq_list_remove(&ctx->iopoll_list, prev, pos);
> > +			do_io_reissue(req, -EIO);
> > +			break;
> > +		}
> > +
> 
> That way you'll avoid redundant checks for REQ_F_REISSUE here.

Another do_io_reissue() needn't to remove req from ->iopoll_list, that
is why the check is done here.


Thanks,
Ming
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Re: [dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Mike Snitzer]

On Tue, Apr 05 2022 at 10:09P -0400,
Ming Lei <ming.lei@redhat.com> wrote:

> On Mon, Apr 04, 2022 at 12:51:30PM -0400, Mike Snitzer wrote:
> > On Sun, Apr 03 2022 at  7:45P -0400,
> > Ming Lei <ming.lei@redhat.com> wrote:
> > 
> > > -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is
> > > set in io_complete_rw_iopoll(), but the req never gets chance to be handled.
> > > io_iopoll_check doesn't handle this situation, and io hang can be caused.
> > > 
> > > Current dm io polling may return -EAGAIN after bio submission is
> > > returned, also blk-throttle might trigger this situation too.
> > > 
> > > Cc: Mike Snitzer <snitzer@kernel.org>
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > 
> > I first reverted commit 5291984004ed ("dm: fix bio polling to handle
> > possibile BLK_STS_AGAIN") then applied this patch and verified this
> > fixes the DM bio polling hangs.  Nice work!
> > 
> > But interestingly with this fio test (against dm-linear ontop of
> > null_blk with queue_mode=2 submit_queues=8 poll_queues=2 bs=4096 gb=16):
> > 
> > fio --bs=4096 --ioengine=io_uring --fixedbufs --registerfiles --hipri=1 \
> > --iodepth=16 --iodepth_batch_submit=16 --iodepth_batch_complete_min=16 \
> > --filename=/dev/mapper/linear --direct=1 --runtime=20 --numjobs=16 \
> > --rw=randread --name=test --group_reporting --norandommap
> 
> 16jobs in io_uring/aio test is overkill.

Sure, it is.. I was just exhausting driver resources.. could fiddle
with it so that it exhausts quickly with even a single thread.

Besides the point really.

> > I get 3186k IOPS with your patch to have io_uring retry (and commit
> > 5291984004ed reverted), but 4305k IOPS if leave commit 5291984004ed
> > applied (and DM resorts to retrying any -EAGAIN _without_ polling).
> 
> IMO, commit 5291984004ed shouldn't be reverted, which is reasonable to
> retry on underlying IO for dm.

Right, I wasn't saying commit 5291984004ed should be reverted.  But I
was testing to see if your patch covered the case commit 5291984004ed
handles.

Note that the DM retry isn't in terms of polling though, polling gets
disabled when the bio is requeued to DM.

> This patch is for making io_uring more reliable, since the current
> io_uring code only handles -EAGAIN from submission code path, and
> -EAGAIN/REISSUE isn't handled if it is returned during ->poll(),
> then the io hang is caused.
> 
> Jens, what do you think of this patch? Does io_uring need to handle
> -EAGAIN in this case?
> 
> > 
> > Jens rightly pointed out to me that polling tests that exhaust tags
> > are bogus anyway (because such unbounded IO defeats the point of
> > polling).  Jens also thinks my result, with commit 5291984004ed
> > applied, is somehow bogus and not to be trusted ;)  He is very likely
> > correct, and the failing likely in the null_blk driver -- I'm
> > skeptical of that driver given it cannot pass fio verify testing
> > (e.g. --do_verify=1 --verify=crc32c --verify_async=1) with or without
> > polling.
> 
> Because it is null block...

Ha, yes.. very good point. I was expecting null_blk capability (read
back written data) that it was never intended to provide. Sorry ;)

> > 
> > Review comments inlined below.
> > 
> > > ---
> > >  fs/io-wq.h    |  13 +++++
> > >  fs/io_uring.c | 128 ++++++++++++++++++++++++++++----------------------
> > >  2 files changed, 86 insertions(+), 55 deletions(-)
> > > 
> > > diff --git a/fs/io-wq.h b/fs/io-wq.h
> > > index dbecd27656c7..4ca4863664fb 100644
> > > --- a/fs/io-wq.h
> > > +++ b/fs/io-wq.h
> > > @@ -96,6 +96,19 @@ static inline void wq_list_add_head(struct io_wq_work_node *node,
> > >  	WRITE_ONCE(list->first, node);
> > >  }
> > >  
> > > +static inline void wq_list_remove(struct io_wq_work_list *list,
> > > +				  struct io_wq_work_node *prev,
> > > +				  struct io_wq_work_node *node)
> > > +{
> > > +	if (!prev)
> > > +		WRITE_ONCE(list->first, node->next);
> > > +	else
> > > +		prev->next = node->next;
> > > +
> > > +	if (node == list->last)
> > > +		list->last = prev;
> > > +}
> > > +
> > >  static inline void wq_list_cut(struct io_wq_work_list *list,
> > >  			       struct io_wq_work_node *last,
> > >  			       struct io_wq_work_node *prev)
> > > diff --git a/fs/io_uring.c b/fs/io_uring.c
> > > index 59e54a6854b7..6db5514e10ca 100644
> > > --- a/fs/io_uring.c
> > > +++ b/fs/io_uring.c
> > > @@ -2759,6 +2759,65 @@ static inline bool io_run_task_work(void)
> > >  	return false;
> > >  }
> > >  
> > > +#ifdef CONFIG_BLOCK
> > > +static bool io_resubmit_prep(struct io_kiocb *req)
> > > +{
> > > +	struct io_async_rw *rw = req->async_data;
> > > +
> > > +	if (!req_has_async_data(req))
> > > +		return !io_req_prep_async(req);
> > > +	iov_iter_restore(&rw->s.iter, &rw->s.iter_state);
> > > +	return true;
> > > +}
> > > +
> > > +static bool io_rw_should_reissue(struct io_kiocb *req)
> > > +{
> > > +	umode_t mode = file_inode(req->file)->i_mode;
> > > +	struct io_ring_ctx *ctx = req->ctx;
> > > +
> > > +	if (!S_ISBLK(mode) && !S_ISREG(mode))
> > > +		return false;
> > > +	if ((req->flags & REQ_F_NOWAIT) || (io_wq_current_is_worker() &&
> > > +	    !(ctx->flags & IORING_SETUP_IOPOLL)))
> > > +		return false;
> > > +	/*
> > > +	 * If ref is dying, we might be running poll reap from the exit work.
> > > +	 * Don't attempt to reissue from that path, just let it fail with
> > > +	 * -EAGAIN.
> > > +	 */
> > > +	if (percpu_ref_is_dying(&ctx->refs))
> > > +		return false;
> > > +	/*
> > > +	 * Play it safe and assume not safe to re-import and reissue if we're
> > > +	 * not in the original thread group (or in task context).
> > > +	 */
> > > +	if (!same_thread_group(req->task, current) || !in_task())
> > > +		return false;
> > > +	return true;
> > > +}
> > > +#else
> > > +static bool io_resubmit_prep(struct io_kiocb *req)
> > > +{
> > > +	return false;
> > > +}
> > > +static bool io_rw_should_reissue(struct io_kiocb *req)
> > > +{
> > > +	return false;
> > > +}
> > > +#endif
> > > +
> > > +static void do_io_reissue(struct io_kiocb *req, int ret)
> > > +{
> > > +	if (req->flags & REQ_F_REISSUE) {
> > > +		req->flags &= ~REQ_F_REISSUE;
> > > +		if (io_resubmit_prep(req))
> > > +			io_req_task_queue_reissue(req);
> > > +		else
> > > +			io_req_task_queue_fail(req, ret);
> > > +	}
> > > +}
> > 
> > Minor nit but: I'd leave caller to check for REQ_F_REISSUE.
> > 
> > > +
> > > +
> > >  static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> > >  {
> > >  	struct io_wq_work_node *pos, *start, *prev;
> > > @@ -2786,6 +2845,13 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> > >  		if (READ_ONCE(req->iopoll_completed))
> > >  			break;
> > >  
> > > +		/*
> > > +		 * Once REISSUE flag is set, the req has been done, and we
> > > +		 * have to retry
> > > +		 */
> > > +		if (req->flags & REQ_F_REISSUE)
> > > +			break;
> > > +
> > >  		ret = kiocb->ki_filp->f_op->iopoll(kiocb, &iob, poll_flags);
> > >  		if (unlikely(ret < 0))
> > >  			return ret;
> > > @@ -2807,6 +2873,12 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, bool force_nonspin)
> > >  	wq_list_for_each_resume(pos, prev) {
> > >  		struct io_kiocb *req = container_of(pos, struct io_kiocb, comp_list);
> > >  
> > > +		if (req->flags & REQ_F_REISSUE) {
> > > +			wq_list_remove(&ctx->iopoll_list, prev, pos);
> > > +			do_io_reissue(req, -EIO);
> > > +			break;
> > > +		}
> > > +
> > 
> > That way you'll avoid redundant checks for REQ_F_REISSUE here.
> 
> Another do_io_reissue() needn't to remove req from ->iopoll_list, that
> is why the check is done here.

All do_io_reissue callers would need to first check for REQ_F_REISSUE,
but my point (purely about avoiding redundant checks) is moot if this
patch isn't safe.

Mike

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Re: [dm-devel] [RFC PATCH] io_uring: reissue in case -EAGAIN is returned after io issue returns

[Bryn M. Reeves]

On Wed, Apr 06, 2022 at 12:52:11PM -0400, Mike Snitzer wrote:
> On Tue, Apr 05 2022 at 10:09P -0400, Ming Lei <ming.lei@redhat.com> wrote:
> > Because it is null block...
> 
> Ha, yes.. very good point. I was expecting null_blk capability (read
> back written data) that it was never intended to provide. Sorry ;)

You can configure a memory backed nullblk instance using the configfs
interface that will return written data on reads:

  # modprobe null_blk nr_devices=0
  # mkdir -p /sys/kernel/config/nullb/nullb0
  # echo 500 > /sys/kernel/config/nullb/nullb0/size
  # echo 1 > /sys/kernel/config/nullb/nullb0/memory_backed
  # echo 1 > /sys/kernel/config/nullb/nullb0/power 
  # lsblk | grep null
  nullb0               250:0    0  500M  0 disk 

It's good enough to make and mount an ext4 file system; the contents
persist across umount as you'd expect and the image will pass an fsck:

  # mkfs.ext4 /dev/nullb0 
  mke2fs 1.46.3 (27-Jul-2021)
  ...

However there's still something in null_blk that causes fio's verify
mode some indigestion:

  # fio --bs=4096 --ioengine=io_uring --fixedbufs --registerfiles --hipri=1 --iodepth=2 --iodepth_batch_submit=2 --iodepth_batch_complete_min=2 --filename=/dev/nullb0 --direct=1 --runtime=20 --numjobs=2 --rw=randread --name=test --group_reporting --norandommap --do_verify=1 --verify=crc32c --verify_async=1
  test: (g=0): rw=randread, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=2
  ...
  fio-3.26
  Starting 2 processes
  fio: filehash.c:64: __lookup_file_hash: Assertion `f->fd != -1' failed.
  verify: bad magic header 0, wanted acca at file /dev/nullb0 offset 31621120, length 4096 (requested block: offset=31621120, length=4096)
  verify: bad magic header 0, wanted acca at file /dev/nullb0 offset 388124672, length 4096 (requested block: offset=388124672, length=4096)
  fio: pid=2074, err=84/file:verify.c:1444, func=async_verify, error=Invalid or incomplete multibyte or wide character
  fio: pid=2075, got signal=6
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  Jobs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  
  ^Cbs: 1 (f=0): [f(1),K(1)][-.-%][eta 00m:00s]
  fio: terminating on signal 2

I haven't looked into it any further at this point (I was looking at
null_blk to evaluate it for use in stacker, as a backing device for
test stacks).

Regards,
Bryn.
 

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