Re: [PATCH 1/2] loop: always return block size in LOOP_GET_STATUS

[Omar Sandoval <osandov@osandov.com>]

On Fri, Aug 18, 2017 at 09:18:52AM +0200, Hannes Reinecke wrote:
> On 08/18/2017 08:15 AM, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@fb.com>
> > 
> > When I was writing a test for the new loop device block size
> > functionality, I realized that the interface is kind of dumb:
> > 
> > - lo_init[0] is never filled in with the logical block size we
> >   previously set
> > - lo_flags returned from LOOP_GET_STATUS will have LO_FLAGS_BLOCKSIZE
> >   set if we previously called LOOP_SET_STATUS with LO_FLAGS_BLOCKSIZE
> >   set, which doesn't really mean anything
> > 
> > Instead, for LOOP_GET_STATUS, let's always fill in lo_init[0] and set
> > the LO_FLAGS_BLOCKSIZE flag to indicate we support it.
> > 
> > Signed-off-by: Omar Sandoval <osandov@fb.com>
> > ---
> >  drivers/block/loop.c | 33 +++++++++++++++++----------------
> >  1 file changed, 17 insertions(+), 16 deletions(-)
> > 
> Phew. 'Dumb interface'.
> 'tis wasn't me who designed the interface;
> Backwards compability are the watchwords here.
> Personally, I would have loved to design a new interface.
> 
> I've got quite some flak for daring to break existing interfaces, most
> notably setting logical and physical blocksize per default (which I
> would _love_ to have done, seeing that it really makes sense here).
> But as this would change the behaviour I've gone through pains (and
> several _years_ of iterations) to get this sorted.
> 
> So if you  design a blocktest for that ensure that
> a) the sysfs attributes before and after the patch are _identical_
> b) the sysfs attributes will only change if the 'LO_FLAGS_BLOCKSIZE'
> flag has been set
> and
> c) validate the written blocksizes; this is required to be able to
> install bootloaders there
> 
> This whole interface was designed such that you can prepare bootable
> diskimages for S/390 DASDs, which use a native 4k blocksize.

Hi, Hannes,

Wasn't insulting you at all, the only part of the interface I'm
complaining about is LOOP_GET_STATUS missing information :)