Re: [PATCHv6 2/2] loop: support 4k physical blocksize

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

On Fri, Apr 07, 2017 at 11:52:27AM +0200, Hannes Reinecke wrote:
> On 04/07/2017 11:38 AM, Hannes Reinecke wrote:
> > On 04/07/2017 11:34 AM, Ming Lei wrote:
> >> On Fri, Apr 07, 2017 at 08:58:58AM +0200, Hannes Reinecke wrote:
> >>> When generating bootable VM images certain systems (most notably
> >>> s390x) require devices with 4k blocksize. This patch implements
> >>> a new flag 'LO_FLAGS_BLOCKSIZE' which will set the physical
> >>> blocksize to that of the underlying device, and allow to change
> >>> the logical blocksize for up to the physical blocksize.
> >>
> >> Actually this UAPI flag is only for setting logical block size. 
> >>
> > Hmm? No, we're setting the physical blocksize, too.
> > Or am I missing something?
> > 
> >>>
> >>> Signed-off-by: Hannes Reinecke <hare@suse.com>
> >>> ---
> >>>  drivers/block/loop.c      | 36 +++++++++++++++++++++++++++++++-----
> >>>  drivers/block/loop.h      |  1 +
> >>>  include/uapi/linux/loop.h |  3 +++
> >>>  3 files changed, 35 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> >>> index 81b3900..f098681 100644
> >>> --- a/drivers/block/loop.c
> >>> +++ b/drivers/block/loop.c
> >>> @@ -221,7 +221,8 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
> >>>  }
> >>>  
> >>>  static int
> >>> -figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit)
> >>> +figure_loop_size(struct loop_device *lo, loff_t offset, loff_t sizelimit,
> >>> +		 loff_t logical_blocksize)
> >>>  {
> >>>  	loff_t size = get_size(offset, sizelimit, lo->lo_backing_file);
> >>>  	sector_t x = (sector_t)size;
> >>> @@ -233,6 +234,12 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
> >>>  		lo->lo_offset = offset;
> >>>  	if (lo->lo_sizelimit != sizelimit)
> >>>  		lo->lo_sizelimit = sizelimit;
> >>> +	if (lo->lo_flags & LO_FLAGS_BLOCKSIZE) {
> >>> +		lo->lo_logical_blocksize = logical_blocksize;
> >>> +		blk_queue_physical_block_size(lo->lo_queue, lo->lo_blocksize);
> >>> +		blk_queue_logical_block_size(lo->lo_queue,
> >>> +					     lo->lo_logical_blocksize);
> >>> +	}
> >>
> >> We can move setting physical block size into loop_set_fd(), and set
> >> 512 bytes as default logical block size in loop_set_fd() too.
> >>
> > Okay.
> > 
> After thinking about it some more I'm not sure if I agree with that
> reasoning.
> 
> One of the goals of this patchset is to keep compability with existing
> installations.
> If we move setting the physical blocksize into loop_set_fd() (which
> needs to be called before loop_set_status()) the physical blocksize
> would always be set.
> Which would induce a user-visible change.
> 
> Hence I've formulated my patch to _not_ change the default setup if the
> new flag isn't set.

OK, better to not break current users, :-)

Thanks,
Ming