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

Linus Torvalds
2007-10-17T17:53:35Z

On Wed, 17 Oct 2007, Jens Axboe wrote:
>
> 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?

I really don't think this should work.

Doing "sg_next()" on a valid sg is *always* ok. So if the old code didn't
work, then "sg" wasn't valid to start with (and the code *after* the
sg_next() would have oopsed even if you try to avoid using sg_next.

So avoiding the "sg_next()" on the last entry is pointless.

Also, your patch makes the code almost totally unreadable, with that
subtle issue of the "if (bvprv && cluster)" case not triggering on the
first case, so the NULL initial sg is "safe".

So at a guess, I think the *real* problem is simply that the passed-in
sglist was just too small. What guarantees that the sg list allocation
(apparently done by scsi_alloc_sgtable()) is big enough?

If I read things right, scsi_alloc_sgtable() will allocate "cmd->use_sg"
SG enties, no? But I also notice that it does not seem to initialize the
SG allocation, so those SG entries contain random crap - including,
perhaps, a random - and bogus - chain pointer in sg->page..

Yes, we set sh->page *if* we create a chain, but if we don't chain, we
leave the old random contents around which in turn may include old and
stale chain pointers. Or am I missing something?

So when you added that "memset(sg, 0, sizeof(*sg))" into blk_rq_map_sg(),
you did it way too late - it needs to be done when the sg chain is
allocated, and for every entry (and then the "link" entry needs to be
linked in separately)

I think.

Linus