Re: [PATCH] SPARC64: fix iommu sg chaining

[Jens Axboe]

On Wed, Oct 17 2007, Jens Axboe wrote:
> On Wed, Oct 17 2007, David Miller wrote:
> > From: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > Date: Wed, 17 Oct 2007 11:16:29 +0200
> > 
> > > On Wed, Oct 17 2007, David Miller wrote:
> > > > From: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > > > Date: Wed, 17 Oct 2007 10:45:28 +0200
> > > > 
> > > > > Righto, it's invalid to call sg_next() on the last entry!
> > > > 
> > > > Unfortunately, that's what the sparc64 code wanted to do, this
> > > > transformation in the sparc64 sg chaining patch is not equilavent:
> > > > 
> > > > -	struct scatterlist *sg_end = sg + nelems;
> > > > +	struct scatterlist *sg_end = sg_last(sg, nelems);
> > > >  ...
> > > > -			while (sg < sg_end &&
> > > > +			while (sg != sg_end &&
> > > 
> > > Auch indeed. That'd probably be better as a
> > > 
> > >         do {
> > >                 ...
> > >         } while (sg != sg_end);
> > 
> > Ok, next bug, introduced by this change:
> > 
> > commit f565913ef8a8d0cfa46a1faaf8340cc357a46f3a
> > Author: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > Date:   Fri Sep 21 10:44:19 2007 +0200
> > 
> >     block: convert to using sg helpers
> >     
> >     Convert the main rq mapper (blk_rq_map_sg()) to the sg helper setup.
> >     
> >     Signed-off-by: Jens Axboe <jens.axboe@xxxxxxxxxx>
> > 
> > Specifically this part:
> > 
> >  new_segment:
> > -			memset(&sg[nsegs],0,sizeof(struct scatterlist));
> > -			sg[nsegs].page = bvec->bv_page;
> > -			sg[nsegs].length = nbytes;
> > -			sg[nsegs].offset = bvec->bv_offset;
> > +			sg = next_sg;
> > +			next_sg = sg_next(sg);
> >  
> > +			sg->page = bvec->bv_page;
> > +			sg->length = nbytes;
> > +			sg->offset = bvec->bv_offset;
> > 
> > You can't remove that memset(), it's there for a reason.  The IOMMU
> > layers depended upon the code zero'ing out the whole scatterlist
> > struct, there might be more to it than page, length and offset :-)
> 
> I realize that, and I was pretty worried about this specific change. But
> there's only been one piece of fallout because if it until now - well
> two, with the sparc64 stuff.
> 
> The problem is that you cannot zero the entire sg entry, because then
> you'd potentially overwrite the chain pointer.
> 
> I'd propose just adding a
> 
>         sg_dma_address(sg) = 0;
>         sg_dma_len(sg) = 0;
> 
> there for now, or provide an arch_clear_sg_entry() helper if we need
> more killed.

Actually, just clearing AFTER sg_next() would be fine, since we know
that is not a link entry. Duh...

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 9eabac9..1014d34 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1352,6 +1352,7 @@ new_segment:
 			sg = next_sg;
 			next_sg = sg_next(sg);
 
+			memset(sg, 0, sizeof(*sg));
 			sg->page = bvec->bv_page;
 			sg->length = nbytes;
 			sg->offset = bvec->bv_offset;

-- 
Jens Axboe

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