Messages in this thread | | | Date | Thu, 31 Jan 2008 16:48:14 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] [1/9] Handle kernel near memory hole in clear_kernel_mapping |
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On Tue, 29 Jan 2008, Andi Kleen wrote: > This was a long standing obscure problem in the relocatable kernel. The > AMD GART driver needs to unmap part of the GART in the kernel direct mapping to > prevent cache corruption. With the relocatable kernel it is in theory possible > that the separate kernel text mapping straddles that area too. > > Normally it should not happen because GART tends to be >= 2GB, and the kernel > is normally not loaded that high, but it is possible in theory.
This smells fishy.
> .... > > +#define overlaps(as, ae, bs, be) ((ae) >= (bs) && (as) <= (be))
inline function please and a bit more intuituive arrangement of arguments.
> +void __init clear_kernel_mapping(unsigned long address, unsigned long size) > +{ > + int sh = PMD_SHIFT; > + unsigned long kernel = __pa(__START_KERNEL_map); > + > + /* > + * Note that we cannot unmap the kernel itself because the unmapped > + * holes here are always at least 2MB aligned.
That's not enforced. The unmap code just does not split pages.
> + * This just applies to the trailing areas of the 40MB kernel mapping.
How is this ensured, that it only affects the end of the 40MB mapping ?
> + */ > + if (overlaps(kernel >> sh, (kernel + KERNEL_TEXT_SIZE) >> sh, > + __pa(address) >> sh, __pa(address + size) >> sh)) {
This checks:
(kernel_end + 1) >= gart_start && kernel_start <= gart_end
One off error: kernel + KERNEL_TEXT_SIZE needs to be: kernel + KERNEL_TEXT_SIZE - 1
Also there is no sanity check, whether the area is inside real kernel text.
> + printk(KERN_WARNING > + "Kernel mapping at %lx within 2MB of memory hole\n", > + kernel);
> + __clear_kernel_mapping(__START_KERNEL_map+__pa(address), size);
Doh! This is unmapping the wrong place. According to __phys_addr():
paddr = vaddr - __START_KERNEL_map + phys_base;
Transformation:
vaddr = padd + __START_KERNEL_map - phys_base;
Sigh. The clear_kernel_mapping() functionality is just another wreckaged implementation of CPA.
We really should use CPA and have a function to mark entries not present. Actually the DEBUG_PAGEALLOC code has one already. There are sanity checks in the CPA code as well, which can be extended to cover the GART unmap in a safe way.
Thanks, tglx
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