Re: [PATCH] [4/8] CPA: Fix set_memory_x for ioremap

Andi Kleen
2008-02-11T12:45:36Z

> Wrong. We do call __pa() on vmalloc ranges (which is a known
> uncleanliness that we intend to fix),

AFAIK nobody does actually currently. Although I expect sooner
or later someone will try since __ioremap() lost its pgprot argument
that made it so powerful. Best would be probably to stick
in some bugs just to catch that.

> but contrary to your claim the
> result is not "random result". On 64-bit it's guaranteed to have a value
> above ~66 TB on 64-bit and hence fails all the filters later on so it
> has zero practical relevance at the moment.

Note that 64bit EFI passes in a fixmap address (they just call
it efi_ioremap). Fixmaps are in the kernel mapping which __pa() handles
and then this gives a low number likely somewhere in memory
and might well trigger.

> On 32-bit we transform it
> down to somewhere around 1GB - where we check it against the BIOS range
> filters - which again cannot trigger. But I do agree that it's unclean
> and needs fixing up.

Are you sure about this for all possible __PAGE_OFFSET values? e.g. consider
1:3 split. Also there is always relocated kernels where kernels might be loaded
quite high.

>
> static int change_page_attr_addr(struct cpa_data *cpa)
> ...
> unsigned long phys_addr = __pa(address);
>
> which for vmalloc area virtual addresses will indeed yield some really
> high (and invalid) physical address. That address will never trigger
> this check:
>
> if (within(address, HIGH_MAP_START, HIGH_MAP_END))
> address = (unsigned long) __va(phys_addr);

That doesn't check phys_addr at all?

> or this check:
>
> if (within(phys_addr, 0, KERNEL_TEXT_SIZE)) {
>
> so we'll never actuall _use_ that phys_addr.

> and it's on our clean-up
> list. But your patch is not a good cleanup because it just hides the
> underlying weakness.

I never claimed it was a cleanup. It's a fix and a optimization
(don't do unnecessary coherency between direct mapping and other
mappings for clearing X -- this means some innocent pages in the
direct mapping won't get split)

Anyways even if you don't want to fix this clear bug I would ask
to still consider the optimization independently.

-Andi