Re: [ 04/21] target/pscsi: Fix page increment

["Nicholas A. Bellinger" <nab@linux-iscsi.org>]

On Mon, 2013-03-18 at 13:35 +0800, Asias He wrote:
> On Sat, Mar 16, 2013 at 02:10:22AM +0000, Ben Hutchings wrote:
> > On Tue, 2013-03-12 at 15:44 -0700, Greg Kroah-Hartman wrote:
> > > 3.0-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Asias He <asias@redhat.com>
> > > 
> > > commit 472b72f2db7831d7dbe22ffdff4adee3bd49b05d upstream.
> > > 
> > > The page++ is wrong. It makes bio_add_pc_page() pointing to a wrong page
> > > address if the 'while (len > 0 && data_len > 0) { ... }' loop is
> > > executed more than one once.
> > > 
> > > Signed-off-by: Asias He <asias@redhat.com>
> > > Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > 
> > > ---
> > >  drivers/target/target_core_pscsi.c |    1 -
> > >  1 file changed, 1 deletion(-)
> > > 
> > > --- a/drivers/target/target_core_pscsi.c
> > > +++ b/drivers/target/target_core_pscsi.c
> > > @@ -1210,7 +1210,6 @@ static int __pscsi_map_task_SG(
> > >  				bio = NULL;
> > >  			}
> > >  
> > > -			page++;
> > >  			len -= bytes;
> > >  			data_len -= bytes;
> > >  			off = 0;
> > 
> > So in case a fragment crosses a page boundary, we wrap around to the
> > beginning of the same page?  That doesn't look right.
> 
> If the fragment crosses a page boundary, what is the correct page
> for it?
> 
> Nicholas, can we assume sg->length + sg->offset should be less than PAGE_SIZE here?
> 

sg->length + sg->offset can be less than or equal to PAGE_SIZE here.

For everything other than tcm_loop + tcm_vhost using externally
allocated SGLs, we can expect fragments to never cross the page
boundary.

For tcm_loop + tcm_vhost, there are a few special cases with control CDB
paylaods (usually going through scsi-generic) where we can have a non
zero sg->offset, but at least in the cases I've seen this is still not
using SGL elements that exceed PAGE_SIZE.

So, I think this logic is OK for SGLs that cross page boundries, given
that it's done outside of the inner loop where *page is set during each
for_each_sg().  The case where this logic is broken, and that the 'page
++' was addressing is when sg->length > PAGE_SIZE.

--nab