Re: [PATCH 4/4] Test chunk size against both origin and snapshot sector size

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Mar 15 2010 at 11:10am -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Mon, 15 Mar 2010, Mike Snitzer wrote:
> 
> > On Mon, Mar 15 2010 at  2:04am -0400,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > Test chunk size against both origin and snapshot sector size
> > > 
> > > Don't allow chunk size smaller than either origin or snapshot logical
> > > sector size. Reading or writing data unaligned to sector size is not allowed
> > > and causes immediate errors.
> > > 
> > > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > > 
> > > ---
> > >  drivers/md/dm-exception-store.c |    4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > Index: linux-2.6.34-rc1-devel/drivers/md/dm-exception-store.c
> > > ===================================================================
> > > --- linux-2.6.34-rc1-devel.orig/drivers/md/dm-exception-store.c	2010-03-12 14:38:31.000000000 +0100
> > > +++ linux-2.6.34-rc1-devel/drivers/md/dm-exception-store.c	2010-03-12 14:39:56.000000000 +0100
> > > @@ -173,7 +173,9 @@ int dm_exception_store_set_chunk_size(st
> > >  
> > >  	/* Validate the chunk size against the device block size */
> > >  	if (chunk_size %
> > > -	    (bdev_logical_block_size(dm_snap_cow(store->snap)->bdev) >> 9)) {
> > > +	    (bdev_logical_block_size(dm_snap_cow(store->snap)->bdev) >> 9) ||
> > > +	    chunk_size %
> > > +	    (bdev_logical_block_size(dm_snap_origin(store->snap)->bdev) >> 9)) {
> > >  		*error = "Chunk size is not a multiple of device blocksize";
> > >  		return -EINVAL;
> > >  	}
> > 
> > Shouldn't we split these checks out so that we can have more precise
> > error reporting?  Ideally we'd share that chunk_size was not a multiple
> > of the "origin" or "snapshot" device's blocksize.
> 
> You can split it to three messages ("not multiple of origin ... snapshot 
> ... both devices' blocksize"), but I think it's not so important to be 
> worth code size increase.
> 
> > I was also thinking that we should avoid using %, e.g.: 
> > (chunk_size & (bdev_logical_block_size(...) - 1))
> > 
> > but AFAIK bdev_logical_block_size() may not be a power of 2 (MD allows
> > for obscure non-power of 2 blocksizes doesn't it?  Or is that just for
> > MD chunk and stripe size?).
> > 
> > Mike
> 
> The Linux bio stack and page cache require that bdev_logical_block_size() 
> is power of two.

OK, I'll have a look, but it sounds like we could use:
(chunk_size & (bdev_logical_block_size(...) - 1))

> But the disks can be reformatted to other block sizes. 
> I'm wondering, what happens then ... I suppose it wouldn't even allow to 
> use the disk. I will try.

Do you mean something like 512b logical and 4K physical?  Such devices
must perform the appropriate r-m-w.  A 4K formatted device will report
4K for both logical and physical (unless the device and format tool
allows for physical != logical).

Mike