On Wed, Oct 17 2007, Jens Axboe wrote:
quoted text
> On Wed, Oct 17 2007, Jens Axboe wrote:
> > On Wed, Oct 17 2007, Linus Torvalds wrote:
> > > 
> > > 
> > > On Wed, 17 Oct 2007, Ingo Molnar wrote:
> > > > 
> > > > Jens, just got this crash on a testbox:
> > > 
> > > The code in question is:
> > > 
> > >      mov    %edx,0xc(%esp)
> > >      mov    (%ebx),%edi
> > >      mov    %edi,%edx
> > >      sub    %eax,%edx
> > >      mov    %edx,%eax
> > >      sar    __PLACEHOLDER__0_x5,%eax
> > >      shl    __PLACEHOLDER__0_xc,%eax
> > >      add    0x8(%ebx),%eax
> > >      cmp    %eax,0xc(%esp)
> > >      je     +126
> > >      mov    0x10(%esi),%eax	<----- Oops
> > >      lea    0x10(%esi),%edx
> > >      test   __PLACEHOLDER__0_x1,%al
> > >      jne    +76
> > >      mov    %edi,(%esi)
> > >      mov    %ebp,0xc(%esi)
> > >      mov    0x8(%ebx),%eax
> > >      mov    %eax,0x4(%esi)
> > > 
> > > 
> > > and it looks like %esi is overflowing from one page to the next one, ie:
> > > 
> > > 	BUG: unable to handle kernel paging request at virtual address 7ca76000
> > > 	ESI: 7ca75ff0
> > > 
> > > and you caught this thanks to page-alloc debugging again.
> > > 
> > > I think I can match that up with the source code: that's "sg_next()". It's 
> > > doing:
> > > 
> > >         sg++;
> > > 
> > >         if (unlikely(sg_is_chain(sg)))
> > >                 sg = sg_chain_ptr(sg);
> > > 
> > >         return sg;
> > > 
> > > and the oopsing instruction is that load of "sg->page" in the assembly 
> > > code:
> > > 
> > > 	mov    0x10(%esi),%eax		# %eax = sg->page
> > > 	lea    0x10(%esi),%edx		# %edx = sg+1;
> > > 	test   __PLACEHOLDER__0_x1,%al			# if (unlikely(sg_is_chain()))
> > > 	jne    +76
> > > 
> > > Jens?
> > 
> > Yep, that's what I came up with as well - I asked Ingo for a dump in
> > private, but ended up just using ksymoops to decode the line.
> > 
> > The way blk_rq_map_sg() operates is that it ends up doing a
> > 
> >         next_sg = sg_next(sg);
> > 
> > even though sg may be the last entry. Perhaps this is crapping out,
> > although if sg is a valid address, then sg + 1 should be as well.
> > next_sg may end up being crap, in fact it will, but we'll never use that
> > unless there are more entries to fill. And if there is, then both sg and
> > next_sg were valid.
> > 
> > So nothing in for-linus should fix it, I'll try and come up with an
> > alternate way to assign next_sg so it's always valid.
> 
> OK, the below should actually be safe, I don't know why I talked myself
> into the next_sg stuff in the beginning. It's always safe to zero sg,
> since it's a valid entry - nothing to save in ->page. Ingo, does this
> work for you?
> 
> diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> index 9e3f3cc..3935469 100644
> --- a/block/ll_rw_blk.c
> +++ b/block/ll_rw_blk.c
> @@ -1322,8 +1322,8 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
>  		  struct scatterlist *sglist)
>  {
>  	struct bio_vec *bvec, *bvprv;
> -	struct scatterlist *next_sg, *sg;
>  	struct req_iterator iter;
> +	struct scatterlist *sg;
>  	int nsegs, cluster;
>  
>  	nsegs = 0;
> @@ -1333,7 +1333,7 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
>  	 * for each bio in rq
>  	 */
>  	bvprv = NULL;
> -	sg = next_sg = &sglist[0];
> +	sg = NULL;
>  	rq_for_each_segment(bvec, rq, iter) {
>  		int nbytes = bvec->bv_len;
>  
> @@ -1349,8 +1349,10 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
>  			sg->length += nbytes;
>  		} else {
>  new_segment:
> -			sg = next_sg;
> -			next_sg = sg_next(sg);
> +			if (!sg)
> +				sg = sglist;
> +			else
> +				sg = sg_next(sg);
>  
>  			memset(sg, 0, sizeof(*sg));
>  			sg->page = bvec->bv_page;
> 

Scratch that, it cannot work... I'll think up a different approach.

-- 
Jens Axboe

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