Re: mlockall and mmap of IO devices don't mix

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

On Sat, Oct 04, 2003 at 12:02:08PM +0200, Ingo Oeser wrote:
> On Saturday 04 October 2003 11:22, you wrote:
> > On Sat, Oct 04, 2003 at 11:17:03AM +0200, Andi Kleen wrote:
> > > > This check is only done, if it is a valid pfn (pfn_valid()) of a
> > > > present pte.
> > >
> > > pfn_valid is useless, it doesn't handle all IO holes on x86 for examples.
> >
> > Sounds like pfn_valid() is buggy on x86.  It's supposed to definitively
> > indicate whether the PFN is a valid page of ram (and has a valid struct
> > page entry.)  If it doesn't do that, the architecture implementation is
> > wrong.
> 
> Looks like it. But it also has to be fast (see include/asm-i386/mmzone.h) 
> and doesn't even hide the holes in NUMA machines. 

It has to be correct.  We do the following in a hell of a lot of places:

	pfn = pte_pfn(pte);
	if (pfn_valid(pfn)) {
		struct page *page = pfn_to_page(pfn);
		/* do something with page */
	}

basically this type of thing happens in any of the PTE manipulation
functions (eg, copy_page_range, zap_pte_range, etc.)

If pfn_valid is returning false positives, and you happen to mmap() an
area which gives false positives from a user space application, your
kernel will either oops or will corrupt RAM when that application exits.

I believe the comment in mmzone.h therefore is an opinion, and indicates
a concern for performance rather than correctness and stability.

-- 
Russell King (rmk@arm.linux.org.uk)	http://www.arm.linux.org.uk/personal/
      Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
      maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
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