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    SubjectRe: [patch] mm: sparsemem memory_present() memory corruption fix
    On (16/04/08 02:03), Ingo Molnar didst pronounce:
    >
    > finally found it ... the patch below solves the sparsemem crash and the
    > testsystem boots up fine now:
    >
    > mars:~> uname -a
    > Linux mars 2.6.25-rc9-sched-devel.git-x86-latest.git #985 SMP Wed Apr 16
    > 01:37:37 CEST 2008 i686 i686 i386 GNU/Linux
    >
    > yay! :-)
    >

    Very cool :) This fixed the silent lock-up that I was getting when using
    your config as well.

    At a bit of a loss yesterday to explain what was going wrong, I had started
    putting together patches to sanity check memory initialisation at various
    different stages trying to catch where things were going pear-shaped. You
    found the bug before it was done but I finished the basics anyway and posted
    it as "[RFC] Verification and debugging of memory initialisation". Something
    like it may help avoid similar headaches for people who tend to run into
    (or cause) boot problems.

    > ps. anyone who can correctly guess the method with which i found the
    > exact place that corrupted memory will get a free beer next time we
    > meet :-)
    >
    > ------------------------->
    > Subject: mm: sparsemem memory_present() memory corruption fix
    > From: Ingo Molnar <mingo@elte.hu>
    > Date: Wed Apr 16 01:40:00 CEST 2008
    >
    > fix memory corruption and crash on 32-bit x86 systems.
    >
    > if a !PAE x86 kernel is booted on a 32-bit system with more than
    > 4GB of RAM, then we call memory_present() with a start/end that
    > goes outside the scope of MAX_PHYSMEM_BITS.
    >
    > that causes this loop to happily walk over the limit of the
    > sparse memory section map:
    >
    > for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
    > unsigned long section = pfn_to_section_nr(pfn);
    > struct mem_section *ms;
    >
    > sparse_index_init(section, nid);
    > set_section_nid(section, nid);
    >
    > ms = __nr_to_section(section);
    > if (!ms->section_mem_map)
    > ms->section_mem_map = sparse_encode_early_nid(nid) |
    >
    > 'ms' will be out of bounds and we'll corrupt a small amount of memory by
    > encoding the node ID. Depending on what that memory is, we might crash,
    > misbehave or just not notice the bug.
    >
    > the fix is to sanity check anything the architecture passes to sparsemem.
    >
    > this bug seems to be rather old (as old as sparsemem support itself),
    > but the exact incarnation depended on random details like configs,
    > which made this bug more prominent in v2.6.25-to-be.
    >
    > an additional enhancement might be to print a warning about ignored
    > or trimmed memory ranges.
    >
    > Signed-off-by: Ingo Molnar <mingo@elte.hu>
    > ---
    > mm/sparse.c | 10 ++++++++++
    > 1 file changed, 10 insertions(+)
    >
    > Index: linux/mm/sparse.c
    > ===================================================================
    > --- linux.orig/mm/sparse.c
    > +++ linux/mm/sparse.c
    > @@ -149,8 +149,18 @@ static inline int sparse_early_nid(struc
    > /* Record a memory area against a node. */
    > void __init memory_present(int nid, unsigned long start, unsigned long end)
    > {
    > + unsigned long max_arch_pfn = 1ULL << (MAX_PHYSMEM_BITS-PAGE_SHIFT);
    > unsigned long pfn;
    >
    > + /*
    > + * Sanity checks - do not allow an architecture to pass
    > + * in larger pfns than the maximum scope of sparsemem:
    > + */
    > + if (start >= max_arch_pfn)
    > + return;
    > + if (end >= max_arch_pfn)
    > + end = max_arch_pfn;
    > +
    > start &= PAGE_SECTION_MASK;
    > for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION) {
    > unsigned long section = pfn_to_section_nr(pfn);
    >

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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