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SubjectRe: [patch] mm: sparsemem memory_present() memory corruption fix

* Yinghai Lu <yhlu.kernel@gmail.com> wrote:

> On Tue, Apr 15, 2008 at 5:44 PM, Ingo Molnar <mingo@elte.hu> wrote:
> >
> > * Yinghai Lu <yhlu.kernel@gmail.com> wrote:
> >
> > > > + unsigned long max_arch_pfn = 1ULL << (MAX_PHYSMEM_BITS-PAGE_SHIFT);
> > > >
> > > > and also check my analysis whether it is correct and whether it
> > > > matches the reported bug patterns. But otherwise the fix looks like
> > > > a safe fix for v2.6.25-final to me - it only filters out values
> > > > from sparsemem input that are nonsensical in the sparsemem
> > > > framework anyway.
> > >
> >
> > > can you check why find_max_pfn() e820_32.c need to call
> > > memory_present? wonder if it can be removed.
> >
> > this is the only call to memory_present() we do in 32-bit arch setup, so
> > it's required.
> >
> > (the function find_max_pfn() is woefully misnamed, but that's a cleanup
> > - i just fixed this in x86.git.)
>
> 64 bit is calling that via paging_init
> ==>sparse_memory_present_with_active_regions(MAX_NUMNODES).
>
> and
> void __init sparse_memory_present_with_active_regions(int nid)

yeah - 64-bit is different here and it's not affected by the problem
because there SECTION_SIZE_BITS is 27 (==128 MB chunks),
MAX_PHYSADDR_BITS is 40 (== 1 TB) - giving 8192 section map entries.
Once larger than 1 TB 64-bit x86 systems are created MAX_PHYSADDR_BITS
needs to be increased.

The only downside of the current setup on 64-bit is that it wastes 128K
of RAM on the majority of systems. We could perhaps try a shift of 28,
which halves the footprint to 64K of RAM, and which still is good enough
to allow the PCI aperture to remain a hole on most systems. It would
also compress the data-cache footprint of the sparse memory maps.
(without having to use sparsemem-extreme indirection)

Ingo


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