Re: [PATCH] block: allow device to have both virt_boundary_mask and max segment size

[Ming Lei <ming.lei@redhat.com>]

On Mon, Apr 08, 2024 at 07:55:42AM +0200, Christoph Hellwig wrote:
> On Sun, Apr 07, 2024 at 09:19:31PM +0800, Ming Lei wrote:
> > When one stacking device is over one device with virt_boundary_mask and
> > another one with max segment size, the stacking device have both limits
> > set. This way is allowed before d690cb8ae14b ("block: add an API to
> > atomically update queue limits").
> > 
> > Relax the limit so that we won't break such kind of stacking setting.
> 
> No, this is broken as discussed before.  With a virt_boundary_mask
> we create a segment for every page (that is device page, which usually
> but not always is the same as the Linux page size).  If we now also
> limit the segment size, we fail to produce valid I/O.

It isn't now we put the limit, and this way has been done for stacking device
since beginning, it is actually added by commit d690cb8ae14b in v6.9-rc1.

If max segment size isn't aligned with virt_boundary_mask, bio_split_rw()
will split the bio with max segment size, this way still works, just not
efficiently. And in reality, the two are often aligned.

> 
> The problem is that that neither the segment_size nor the
> virtual_boundary should be inherited by a stackable device and we
> need to fix that.

It is one big change with regression risk, which may not be good after -rc3.



Thanks,
Ming