Re: [PATCH 2/9] mm: arm ready for split ptlock

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Mon, Oct 31, 2005 at 11:19:21PM +0100, Jesper Juhl wrote:
> On 10/26/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> > No.  If we're emulating a cmpxchg() on a clean BSS page, this code
> > as it stands today will write to the zero page making it a page which
> > is mostly zero.  Bad news when it's mapped into other processes BSS
> > pages.
> >
> > Changing this for pte_dirty() means that we'll refuse to do a cmpxchg()
> > on a clean BSS page.  The data may compare correctly, but because it
> > isn't already dirty, you'll fail.
> >
> > If we still had it, I'd say you need to use verify_area() to tell the
> > kernel to pre-COW the pages.  However, that got removed a while back.
> >
> 
> Yes, I removed verify_area() since it was just a wrapper for access_ok().
> If verify_area() was/is needed, then access_ok() should be just fine
> as a replacement as far as I can see.

Except verify_area() would pre-fault the pages in whereas access_ok()
just verifies that the address is a user page.  That's quite important
in this case because in order to fault the page in, we need to use
put_user() to get the permission checking correct.

However, we can't use put_user() because then the cmpxchg emulation
becomes completely non-atomic.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core