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

[Linus Torvalds]

On Wed, 17 Oct 2007, Linus Torvalds wrote:
> 
> I think you'll always hit it if you have a scatter-gather list that is 
> exactly filled up.

In fact, I think you'll hit it even if you use the "for_each_sg()" 
helper function. Exactly because it does the sg = sg_next(sg) at the end 
of the for-loop, so it will do it for the last entry too, even though that 
will access one past the end.

So it really *is* the case that every *single* use of that SG chain needs 
to be switched over to only do the sg_next() when required, or sg_next() 
needs to be fixed to look at the current-in-use entry (which we *can* 
access) when it decides what to do about the next one (which we can *not* 
access).

Moving the sg_is_chain() bit to the previous entry would work, but it 
requires that nobody who could have a chained scatterlist must *ever* 
access "sg->page" directly - you'd always need to use some helper function 
that masks off the bit, eg

 - rename "sg->page" into "sh->page_and_flag", and make it "unsigned long" 
   instead of a pointer.

 - change every single "sg->page" access to use "sg_page(sg)" instead:

	#define sg_pointer(sg)	((void *)((sg)->page_and_flag & ~1ul))

	static inline struct page *sg_page(struct scatterist *sg)
	{
		return sg_pointer(sg);
	}

 - change "sg_next()" to do

	static inline struct scatterlist *sg_next(struct scatterlist *sg)
	{
		if (sg->page_and_flag & 1)
			sg = sg_pointer(sg+1)-1;
		return ++sg;
	}

where the magic is exactly the fact that now "sg_next()" will *not* 
derefence the next SG entry unless the current one was marked 
appropriately.

And then *creating* the chain obviously needs to properly mark the 
next-to-last entry with the "next entry is a pointer" flag.

Big diff, it sounds like. But I don't see many alternatives. Jens?

		Linus
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