[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]

Re: [dm-devel] A target to just process bios background?



Hi,

> If I understand correctly, any sync I/O causes deadlock if it's called under .map hook.
> And blkdev_issue_flush() seems to be sync (submitting and wait for the completion after that) but
> never cause deadlock in my environment.
Sorry. I was really confused. That causes deadlock definitely.

> Is it not allowed to call blkdev_issue_flush() directly under .map?
> No target uses the function in device-mapper.
blkdev_issue_flush() is not used in device-mapper at present but instead
some targets uses dm_io with WRITE_FLUSH such as

- flush_header() in dm-log.c
- mirror_flush() in dm-raid1.c

these codes
- set the sector and count of dm_io_region to zero
- set the write buffer is NULL
- and then submits with WRITE_FLUSH flag using dm_io() routine.

the dm-insitu-compression target newly proposed by Shaohua
only uses blkdev_issue_flush() in its insitu_comp_flush_dirty_meta()
but it's called on background and thus, sane.

--
Akira

On Mon, 10 Mar 2014 17:18:07 +0900
Akira Hayakawa <hayakawa valinux co jp> wrote:

> Hi, Mikulas,
> 
> > Such target doesn't exist and I wouldn't recommend to make it. If you 
> > add the bio to a queue and return immediatelly from the map routine, the 
> > caller sends another bio. So, in the end, you end up with extreme amount 
> > of bios on the queue and very bad responsiveness.
> I see. It was just a question. Don't worry, dm-writeboost doesn't contain such a code.
> 
> By the way, I want to ask you a question related to this topic.
> 
> calling dm_io() in sync mode under .map hook causes deadlock.
> To avoid that, for example, dm-snap-persistent.c executes it in a different thread.
> 
>         /*
>          * Issue the synchronous I/O from a different thread
>          * to avoid generic_make_request recursion.
>          */
>         INIT_WORK_ONSTACK(&req.work, do_metadata);
>         queue_work(ps->metadata_wq, &req.work);
>         flush_workqueue(ps->metadata_wq);
>         destroy_work_on_stack(&req.work);
> 
> If we always queue the bio and executes it in a different thread
> we never be suffered from this problem and that's why I call this topic is "related".
> 
> The cause of the deadlock is that the context has only one bio_list,
> the bio submitted under sync dm_io() never be executed the caller bio completes.
> Thus, deadlock.
> 
> If I understand correctly, any sync I/O causes deadlock if it's called under .map hook.
> And blkdev_issue_flush() seems to be sync (submitting and wait for the completion after that) but
> never cause deadlock in my environment.
> 
> Is it not allowed to call blkdev_issue_flush() directly under .map?
> No target uses the function in device-mapper.
> 
> --
> Akira
> 
> On Thu, 6 Mar 2014 17:57:21 -0500 (EST)
> Mikulas Patocka <mpatocka redhat com> wrote:
> 
> > 
> > 
> > On Tue, 4 Mar 2014, Akira Hayakawa wrote:
> > 
> > > Hi,
> > > 
> > > Is it meaningless to submit the split and cloned bios in background
> > > using workqueue?
> > > 
> > > device-mapper doesn't go to next split before the .map hook returns
> > > (and generic_make_request() returns only in case of DM_MAPIO_REMAPPED).
> > > So, quitting from .map hook and going into next split fast sounds to me
> > > effective at least in terms of CPU usage (in multicore system).
> > > 
> > > is this discussed before?
> > > 
> > > A target as tiny as linear or flakey can be thought:
> > > - it has work_struct in per_bio_data
> > > - .map hook queue_work the work into private wq.
> > > - and then return DM_MAPIO_SUBMITTED
> > > 
> > > is this implemented before?
> > > 
> > > I think this target will make people happy if they
> > > want to see what if the bio submission is done background
> > > without changing their code but only stacking a dm target.
> > > 
> > > I am sorry if I am confused.
> > 
> > Hi
> > 
> > Such target doesn't exist and I wouldn't recommend to make it. If you 
> > add the bio to a queue and return immediatelly from the map routine, the 
> > caller sends another bio. So, in the end, you end up with extreme amount 
> > of bios on the queue and very bad responsiveness.
> > 
> > Suppose, for example, that you have a system with 16GB memory. 20% can be 
> > marked dirty (that's the default value for /proc/sys/vm/dirty_ratio), and 
> > if you use 4k bios, you may have 838860 bios on the queue. Any process 
> > that tries to write anything to disk freezes until all of them are 
> > written.
> > 
> > In fact, dm-crypt behaves this way - and we have a bug report that it 
> > causes out-of-memory crashes when massive amounts of bios are added to the 
> > queue.
> > 
> > dm-mirror also behaves this way, but only for write bios - you can load a 
> > mirror target with both legs pointing to the same device if you want to 
> > see how does it behave.
> > 
> > Mikulas
> > 
> > --
> > dm-devel mailing list
> > dm-devel redhat com
> > https://www.redhat.com/mailman/listinfo/dm-devel
> 
> 
> -- 
> Akira Hayakawa <hayakawa valinux co jp>
> 
> --
> dm-devel mailing list
> dm-devel redhat com
> https://www.redhat.com/mailman/listinfo/dm-devel


[Date Prev][Date Next]   [Thread Prev][Thread Next]   [Thread Index] [Date Index] [Author Index]