Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Dec 03 2020 at 10:59pm -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> On Thu, Dec 03, 2020 at 09:03:43PM -0500, Mike Snitzer wrote:
> > On Thu, Dec 03 2020 at  8:12pm -0500,
> > Ming Lei <ming.lei@redhat.com> wrote:
> > 
> > > On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote:
> > > > On Wed, Dec 02 2020 at 10:26pm -0500,
> > > > Ming Lei <ming.lei@redhat.com> wrote:
> > > > 
> > > > > On Tue, Dec 01, 2020 at 11:07:09AM -0500, Mike Snitzer wrote:
> > > > > > commit 22ada802ede8 ("block: use lcm_not_zero() when stacking
> > > > > > chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must
> > > > > > reflect the most limited of all devices in the IO stack.
> > > > > > 
> > > > > > Otherwise malformed IO may result. E.g.: prior to this fix,
> > > > > > ->chunk_sectors = lcm_not_zero(8, 128) would result in
> > > > > > blk_max_size_offset() splitting IO at 128 sectors rather than the
> > > > > > required more restrictive 8 sectors.
> > > > > 
> > > > > What is the user-visible result of splitting IO at 128 sectors?
> > > > 
> > > > The VDO dm target fails because it requires IO it receives to be split
> > > > as it advertised (8 sectors).
> > > 
> > > OK, looks VDO's chunk_sector limit is one hard constraint, even though it
> > > is one DM device, so I guess you are talking about DM over VDO?
> > > 
> > > Another reason should be that VDO doesn't use blk_queue_split(), otherwise it
> > > won't be a trouble, right?
> > > 
> > > Frankly speaking, if the stacking driver/device has its own hard queue limit
> > > like normal hardware drive, the driver should be responsible for the splitting.
> > 
> > DM core does the splitting for VDO (just like any other DM target).
> > In 5.9 I updated DM to use chunk_sectors, use blk_stack_limits()
> > stacking of it, and also use blk_max_size_offset().
> > 
> > But all that block core code has shown itself to be too rigid for DM.  I
> > tried to force the issue by stacking DM targets' ti->max_io_len with
> > chunk_sectors.  But really I'd need to be able to pass in the per-target
> > max_io_len to blk_max_size_offset() to salvage using it.
> > 
> > Stacking chunk_sectors seems ill-conceived.  One size-fits-all splitting
> > is too rigid.
> 
> DM/VDO knows exactly it is one hard chunk_sectors limit, and DM shouldn't play
> the stacking trick on VDO's chunk_sectors limit, should it?

Feel like I already answered this in detail but... correct, DM cannot
and should not use stacked chunk_sectors as basis for splitting.

Up until 5.9, where I changed DM core to set and then use chunk_sectors
for splitting via blk_max_size_offset(), DM only used its own per-target
ti->max_io_len in drivers/md/dm.c:max_io_len().

But I reverted back to DM's pre-5.9 splitting in this stable@ fix that
I'll be sending to Linus today for 5.10-rcX:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10-rcX&id=6bb38bcc33bf3093c08bd1b71e4f20c82bb60dd1

DM is now back to pre-5.9 behavior where it doesn't even consider
chunk_sectors for splitting (NOTE: dm-zoned sets ti->max_io_len though
so it is effectively achieves the same boundary splits via max_io_len).

With that baseline established, what I'm now saying is: if DM, the most
common limits stacking consumer, cannot benefit from stacked
chunk_sectors then what stacked device does benefit?  Could be block
core's stacked chunk_sectors based splitting is good enough for others,
just not yet seeing how.  Feels like it predates blk_queue_split() and
the stacking of chunk_sectors could/should be removed now.

All said, I'm fine with leaving stacked chunk_sectors for others to care
about... think I've raised enough awareness on this topic now ;)

Mike

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