Messages in this thread | | | Date | Wed, 17 Oct 2007 18:19:07 -0700 (PDT) | Subject | Re: [bug] ata subsystem related crash with latest -git | From | David Miller <> |
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From: Linus Torvalds <torvalds@linux-foundation.org> Date: Wed, 17 Oct 2007 18:07:19 -0700 (PDT)
> sg_next() - as it stands now - never actually looks at the SG that its > argument points to: it explicitly *only* looks at the next one. > > That's the bug. If sg_next() looked at the actual *current* sg entry, we > wouldn't have any issues to begin with, and that's what I'm arguing we > should do in the longer run (where "longer run" is defined as "when Jens > does it asap").
What the thing really wants is some kind of indication of state, without having to bloat up the scatterlist structure.
I believe that we have enough of a limited set of accessors to sg->page that we can more aggressively encode things in the lower bits.
I'm thinking of encoding the low two bits of sg->page as follows:
1) bits == 0
then the SG list is linear and sg_next() is sg++
2) bits == 1
the nest SG is an indirect chunk, sg_next() is therefore something like:
next = *((struct scatterlist **)(sg + 1));
3) bits == 2
this is the last entry in the scatterlist, sg_next() is NULL
So for the cases of ARCH_HAS_SG_CHAIN not being set (ie. back compatible), we can do no bit encoding in page->flags and just do sg_next() == sg++, as is done now.
When doing SG chaining, in each non-linear chunk we have to allocate one more pointer past the end of the scatterlist array (instead of a full extra scatterlist entry for the indirect pointer encode). Next, all sg->page accesses have to be guarded to clear the state bits out first.
I don't know, maybe it would work, and would make the loop termination issues easier to handle properly. - 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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