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On Wed, Oct 17 2007, Linus Torvalds wrote: > > > 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. Yeah, I didn't quite understand why if sg was valid, why dereferencing *(sg + 1)->page would crap out :/ > 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". Hmm I think it's quite readable, but perhaps that's just me :-). The first is much cleaner, and the last part just reads 'if sg is not set yet, set to list. otherwise, goto next entry'. > 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.. Right, we allocate an sgtable that will hold ->use_sg entries, which contains request->nr_phys_segments. And that should definitely fit. Regarding the init of the sglist, that was the revert I was talking about. We do need that memset() in there, so all those sg entries will be properly zeroed. > 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. Yep, and that is what Ingo did test as well and it worked. For that case, now libata is crapping out elsewhere in sg_next(). -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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