Re: [PATCH] scsi_debug: disable clustering

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2008-02-18 at 00:11 +0900, FUJITA Tomonori wrote:
> On Sun, 17 Feb 2008 09:02:14 -0600
> James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> 
> > 
> > On Sun, 2008-02-17 at 23:52 +0900, FUJITA Tomonori wrote:
> > > On Sun, 17 Feb 2008 07:28:48 -0700
> > > Matthew Wilcox <matthew@wil.cx> wrote:
> > > 
> > > > On Sun, Feb 17, 2008 at 08:18:11AM -0600, James Bottomley wrote:
> > > > > No, he means that kmap_atomic can only map a page of data.  This makes
> > > > > single page only sg list entries and input assumption into this loop.
> > > > > with ENABLE_CLUSTERING, that's potentially not true.   Of course, this
> > > > > accidentally works most of the time because of the way kmap functions.
> > > > 
> > > > Ah, right.  I'm on the verge of releasing a ram-based scsi driver I've
> > > > been working on ... this loop should work fine with clustering as it
> > > > takes account of the sg potentially having multiple pages:
> > > > 
> > > >         scsi_for_each_sg(cmnd, sg, scsi_sg_count(cmnd), i) {
> > > >                 struct page *sgpage = sg_page(sg);
> > > >                 unsigned int to_off = sg->offset;
> > > >                 unsigned int sg_copy = sg->length;
> > > >                 if (sg_copy > len)
> > > >                         sg_copy = len;
> > > >                 len -= sg_copy;
> > > 
> > > stex driver has a similar function to copies data between a buffer and
> > > a scatter list. I think that scsi_kmap_atomic_sg is a bit primitive
> > > (and not very popular).  I'll send a patch to add a helper function to
> > > scsi_lib.c that copies data between a buffer and a scatter list. It
> > > would be useful for several drivers.
> > 
> > Actually, if you're going to sweep up them all, libata also does this.
> 
> Thanks, I'll look at it.
> 
> 
> > However, mapping and copying data isn't a SCSI specific function, it's
> > one any virtual block driver should do, so I think block might be the
> > correct location for such a function.
> 
> I see. It could go to lib/scatterlist.c or block/ I guess.

Actually, lib/scatterlist is probably better.

James