Re: [PATCH] Re: swapcache bug?

[Linus Torvalds <torvalds@transmeta.com>]

On Mon, 8 Feb 1999, Stephen C. Tweedie wrote:
> 
> Good point, the line include/linux/pagemap.h:39,
> 
> 	return s(i+o) & (PAGE_HASH_SIZE-1);
> 
> should probably be 
> 
> 	return s(i+o+offset) & (PAGE_HASH_SIZE-1);
> 
> to mix in the low order bits for swap entries.  Well spotted.  Anyone
> see anything wrong with this one-liner change?

Yes, the above will potentially result in different hash entries for the
same page, which means that we now have aliasing and basically just random
behaviour. 

It _may_ be that the hash function is always called with a page-aligned
offset, but that was not how it was strictly meant to be: the way the
thing was envisioned you could just find the page at "offset" by doing

	page_hash(inode,offset)

without page-aligning offset before you did this.

If anything, maybe the swap cache should just use the high bits in the
"offset" field (or at least prefer to do so: something like

	page->offset = swap_entry_to_offset(entry);

and 
	entry = offset_to_swap_entry(page->offset);

that does a PAGE_MASK_BITS rotate on the bits..

		Linus

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