Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)

[Linus Torvalds]

On Tue, 5 Mar 2002, Rusty Russell wrote:
>
> 1) FUTEX_UP and FUTEX_DOWN defines. (Robert Love)
> 2) Fix for the "decrement wraparound" problem (Paul Mackerras)
> 3) x86 fixes: tested on dual x86 box.

This doesn't work on highmem machines - doing the conversion from "<struct
page, offset>" to "page_address(page)+offset" is simply not legal (not
even for pure hashing purposes - page_address() changes as you kmap it).

You need to keep the <struct page,offset> tuple in that format, and no
other. And when you actually touch the page, you need to do the
kmap()/kunmap() (and you must not keep it mapped while you sleep, because
that might trivially make the kernel run out of virtual mappings).

		Linus