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

Jens Axboe
2007-10-17T17:29:51Z

On Wed, Oct 17 2007, Jens Axboe wrote:
> On Wed, Oct 17 2007, Jens Axboe wrote:
> > 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;
> >
>
> Scratch that, it cannot work... I'll think up a different approach.

OK, it is fine, as long as the sglist is cleared initially. And I don't
think there's anyway around that, clearly I didn't think long enough
before including the memset() removal from Tomo.

Ingo, please try this rolled up version.

Linus, this should work. It would probably be best if you first did a
git revert on f5c0dde4c66421a3a2d7d6fa604a712c9b0744e5 and then applied
the ll_rw_blk.c bit alone. Do you want me to stuff that (revert + patch)
into a branch for you to pull?

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;
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0c86be7..aac8a02 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -764,6 +764,8 @@ struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
if (unlikely(!sgl))
goto enomem;

+ memset(sgl, 0, sizeof(*sgl) * sgp->size);
+
/*
* first loop through, set initial index and return value
*/

--
Jens Axboe