Re: [PATCH] dm-delay: support zoned devices

[Benjamin Marzinski <bmarzins@redhat.com>]

On Wed, Mar 26, 2025 at 11:17:24AM -0400, Damien Le Moal wrote:
> On 2025/03/26 11:00, Benjamin Marzinski wrote:
> > On Wed, Mar 26, 2025 at 08:55:48AM -0400, Damien Le Moal wrote:
> >> On 2025/03/21 13:52, Benjamin Marzinski wrote:
> >>> On Fri, Mar 21, 2025 at 08:18:16AM +0100, Christoph Hellwig wrote:
> >>>> Add support for zoned device by passing through report_zoned to the
> >>>> underlying read device.
> >>>>
> >>>> This is required to make enable xfstests xfs/311 on zoned devices.
> >>>
> >>> On suspend, delay_presuspend() stops delaying and it doesn't guarantee
> >>> that new bios coming in will always be submitted after the delayed bios
> >>> it is flushing. That can mess things up for zoned devices. I didn't
> >>> check if that matters for the specific test. Setting
> >>>
> >>> ti->emulate_zone_append = true;
> >>>
> >>> would enforce write ordering, at the expense of adding a whole other
> >>> layer of delays to zoned dm-delay devices. Since this isn't really
> >>> useful outside of testing, I think that could be acceptable if necessary
> >>> (it would require us to support table reloads of zoned devices with
> >>> emulated zone append, since tests often want to change the delay).
> >>> However it would probably be better to see if we can just make dm-delay
> >>> preserve write ordering during a suspend.
> >>
> >> delay_presuspend() calls flush_delayed_bios() with flush_all == true. So all
> >> BIOs will be flushed in the order they are queued in the delay list, which as
> >> far as I can tell is the order in which the user of dm-delay issued the BIOs. So
> >> for writes, the order is preserved as far as I can tell.
> > 
> > delay_presuspend() is called before we set the DMF_BLOCK_IO_FOR_SUSPEND
> > bit, which will stop incoming bio from getting mapped, and also before
> > lock_fs() is called. This means it's common for new bios to continue to
> > come into delay_map(), while delay_presuspend() is running.  The moment
> > delay_presuspend() sets dc->may_delay = false, those new bios will stop
> > getting queued by delay_bio(). They will get remapped immeditately to
> > the underlying device. flush_delayed_bios() doesn't even get called
> > until after dc->may_delay is set to false, and if there are a lot of
> > bios on the delayed_bios list, flush_delayed_bios() will schedule. So,
> > it's actually very common for new incoming bios to get passed to
> > underlying device before all the bios on the dc->delayed_bios list do.
> > 
> > Solving this without grabbing the dc->process_bios_lock mutex for every
> > bio sent to dm-delay probably involves keeping the incoming bios going
> > to dc->delayed_bios during suspend, at least until we can guarantee that
> > it's empty and no bios are being flushed.
> 
> OK. Understood. Thank you for the explanation. And the above sounds like a
> rather simple solution, which does not even needs to be zone specific.
> 
> I also think that this is orthogonal to Christoph patch and we can fix the
> suspend issue on top of Christoph's patch. This is a very niche issue anyway for
> the main target use case which is fstests, since fstests does not suspend/resume
> the dm-delay device as far as I know.

I'm not sure that I would call a patch that adds a feature which doesn't
work correctly in some situations orthogonal to making the feature work
correctly in those situations. But I'll grant you that dm-delay is a
testing target, and Christoph's patch makes in useful for more tests, as
long as they don't suspend the device while there is still outstanding
IO. So, sure. I agree that Christoph doesn't need to change his patch,
and we can fix the suspend issue in a separate one. How that all gets
merged is Mikulas's call.

-Ben

> 
> > 
> > -Ben
> > 
> >>
> >> -- 
> >> Damien Le Moal
> >> Western Digital Research
> > 
> 
> 
> -- 
> Damien Le Moal
> Western Digital Research