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 11:27am -0500,
Keith Busch <kbusch@kernel.org> 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:
> > 
> > > I understand it isn't related with correctness, because the underlying
> > > queue can split by its own chunk_sectors limit further. So is the issue
> > > too many further-splitting on queue with chunk_sectors 8? then CPU
> > > utilization is increased? Or other issue?
> > 
> > No, this is all about correctness.
> > 
> > Seems you're confining the definition of the possible stacking so that
> > the top-level device isn't allowed to have its own hard requirements on
> > IO sizes it sends to its internal implementation.  Just because the
> > underlying device can split further doesn't mean that the top-level
> > virtual driver can service larger IO sizes (not if the chunk_sectors
> > stacking throws away the hint the virtual driver provided because it
> > used lcm_not_zero).
> 
> I may be missing something obvious here, but if the lower layers split
> to their desired boundary already, why does this limit need to stack?

The problematic scenario is when the topmost layer, or layers, are the
more constrained.  _That_ is why the top-level's chunk_sectors limit
cannot be relaxed.

For example (in extreme where chunk_sectors is stacked via gcd):
dm VDO target (chunk_sectors=4K)
on dm-thin (ideally chunk_sectors=1280K, reality chunk_sectors=128K)
on 10+2 RAID6 (chunk_sectors=128K, io_opt=1280K)
on raid members (chunk_sectors=0)

Results in the following bottom up blk_stack_limits() stacking:
gcd(128K, 0) = 128K -> but MD just sets chunk_sectors, no stacking is done afaik
gcd(1280K, 128K) = 128K -> this one hurts dm-thin, needless splitting
gcd(4K, 128K) = 4K -> vdo _must_ receive 4K IOs, hurts but "this is the way" ;)

So this is one extreme that shows stacking chunk_sectors is _not_
helpful (if the resulting chunk_sectors were actually used as basis for
splitting).  Better for each layer to just impose its own chunk_sectors
without concern for the layers below.

Think I'd be fine with block core removing the chunk_sectors stacking
from blk_stack_limits()...
(and as you see below, I've been forced to revert to _not_ using stacked
chunk_sectors based splitting in DM)

> Won't it also work if each layer sets their desired chunk_sectors
> without considering their lower layers? The commit that initially
> stacked chunk_sectors doesn't provide any explanation.

Yes, I think it would work.  The current stacking doesn't have the
luxury of knowing which layer a blk_stack_limits() maps too.  BUT within
a layer chunk_sectors really does need to be compatible/symbiotic.  So
it is unfortunately all or nothing as you build up the stack.

And that all-or-nothing stacking of chunk_sectors is why I've now (just
last night, based on further review by jdorminy) had to punt on using
stacked chunk_sectors and revert DM back to doing its own fine-grained
(and varied) splitting on a per DM target basis, see:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10-rcX&id=6bb38bcc33bf3093c08bd1b71e4f20c82bb60dd1

Kind of depressing that I went so far down the rabbit hole, of wanting
to lean on block core, that I lost sight of an important "tenet of DM":

+         * Does the target need to split IO even further?
+         * - varied (per target) IO splitting is a tenet of DM; this
+         *   explains why stacked chunk_sectors based splitting via
+         *   blk_max_size_offset() isn't possible here.

And it is because of this that DM is forced to lean on human creation of
an optimal IO stack.. which is prone to human error when a particular
thinp "blocksize" is selected, etc.

Mike

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