[dm-devel] [PATCH 4/4] Test chunk size against both origin and snapshot sector size

[Mikulas Patocka]

On Mon, 15 Mar 2010, Mike Snitzer wrote:

> On Mon, Mar 15 2010 at  2:04am -0400,
> Mikulas Patocka <mpatocka at 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 at 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. 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.

Mikulas