Re: news about IDE PIO HIGHMEM bug (was: Re: 2.6.9-mm1)

[Jeff Garzik]

Martin J. Bligh wrote:
> > Bartlomiej Zolnierkiewicz wrote:
> >
> > > We have stuct page of the first page and a offset.
> > > We need to obtain struct page of the current page and map it.
> >
> >
> > Opening this question to a wider audience.
> >
> > struct scatterlist gives us struct page*, and an offset+length pair. The struct page* is the _starting_ page of a potentially multi-page run of data.
> >
> > The question:  how does one get struct page* for the second, and successive pages in a known-contiguous multi-page run, if one only knows the first page?
>
>
> If it's a higher order allocation, just page+1 should be safe. If it just
> happens to be contig, it might cross a discontig boundary, and not obey
> that rule. Very unlikely, but possible.


Unfortunately, it's not.

The block layer just tells us "it's a contiguous run of memory", which 
implies nothing really about the allocation size.

Bart and I (and others?) essentially need a "page+1" thing (for 2.4.x 
too!), that won't break in the face of NUMA/etc.

Alternatively (or additionally), we may need to make sure the block 
layer doesn't merge across zones or NUMA boundaries or whatnot.

	Jeff


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