[dm-devel] Re: [PATCH] Fix panic in 2.6 with bounced bio and dm

[Mark Haverkamp]

On Mon, 2005-02-28 at 16:51 +0100, Jens Axboe wrote:
> On Mon, Feb 28 2005, Mark Haverkamp wrote:
> > On Sat, 2005-02-26 at 13:39 +0100, Jens Axboe wrote:
> > > On Fri, Feb 25 2005, Linus Torvalds wrote:
> > > > 
> > > > 
> > > > On Fri, 25 Feb 2005, Andrew Morton wrote:
> > > > > 
> > > > > It seems very weird for dm to be shoving NULL page*'s into the middle of a
> > > > > bio's bvec array, so your fix might end up being a workaround pending a
> > > > > closer look at what's going on in there.
> > > > 
> > > > Yes. I don't see how this patch can be anything but bandaid to hide the 
> > > > real bug. Where do these "non-page" bvec's originate?
> > > 
> > > Yep that's the fishy part, there should not be NULL pages in the middle
> > > (or empty bios, for that matter) submitted for io.
> > > 
> > > Mark, what was the bug that triggered you to write this patch?
> > 
> > It happened when some pages of IO from a dm device were bounced.  It
> > looks to me when bio's are cloned in the dm code to split it for
> > physical devices that only the pointers to pages that apply to that
> > device are copied and th bi_idx is adjusted to point to the start,
> > leaving some NULL pointers at the start of the bio_vec.
> 
> This should fix it.

Wouldn't this potentially create bounce pages that will never be used?

> 
> Signed-off-by: Jens Axboe <axboe at suse.de>
> 
> 
> ===== mm/highmem.c 1.55 vs edited =====
> --- 1.55/mm/highmem.c	2005-01-08 06:44:13 +01:00
> +++ edited/mm/highmem.c	2005-02-28 16:50:59 +01:00
> @@ -425,7 +425,7 @@
>  	 * at least one page was bounced, fill in possible non-highmem
>  	 * pages
>  	 */
> -	bio_for_each_segment(from, *bio_orig, i) {
> +	__bio_for_each_segment(from, *bio_orig, i) {
>  		to = bio_iovec_idx(bio, i);
>  		if (!to->bv_page) {
>  			to->bv_page = from->bv_page;
> 
-- 
Mark Haverkamp <markh at osdl.org>