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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