Re: [bug] block subsystem related crash with latest -git

Jens Axboe
2007-10-17T17:08:34Z

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 $0x5,%eax
> > shl $0xc,%eax
> > add 0x8(%ebx),%eax
> > cmp %eax,0xc(%esp)
> > je +126
> > mov 0x10(%esi),%eax <----- Oops
> > lea 0x10(%esi),%edx
> > test $0x1,%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 $0x1,%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;

--
Jens Axboe