Bug 455434

Backport

Gitweb:    
http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=aeed5fce37196e09b4dac3a1c00d8b7122e040ce
Commit:     aeed5fce37196e09b4dac3a1c00d8b7122e040ce
Parent:     bb78be8397d3b0900af3d717672218ee3ea07985
Author:     Hugh Dickins <hugh>
AuthorDate: Tue May 6 20:49:23 2008 +0100
Committer:  Linus Torvalds <torvalds>
CommitDate: Tue May 6 13:08:58 2008 -0700

    x86: fix PAE pmd_bad bootup warning
    
    Fix warning from pmd_bad() at bootup on a HIGHMEM64G HIGHPTE x86_32.
    
    That came from 9fc34113f6880b215cbea4e7017fc818700384c2 x86: debug pmd_bad();
    but we understand now that the typecasting was wrong for PAE in the previous
    version: pagetable pages above 4GB looked bad and stopped Arjan from booting.
    
    And revert that cded932b75ab0a5f9181ee3da34a0a488d1a14fd x86: fix pmd_bad
    and pud_bad to support huge pages.  It was the wrong way round: we shouldn't
    weaken every pmd_bad and pud_bad check to let huge pages slip through - in
    part they check that we _don't_ have a huge page where it's not expected.
    
    Put the x86 pmd_bad() and pud_bad() definitions back to what they have long
    been: they can be improved (x86_32 should use PTE_MASK, to stop PAE thinking
    junk in the upper word is good; and x86_64 should follow x86_32's stricter
    comparison, to stop thinking any subset of required bits is good); but that
    should be a later patch.
    
    Fix Hans' good observation that follow_page() will never find pmd_huge()
    because that would have already failed the pmd_bad test: test pmd_huge in
    between the pmd_none and pmd_bad tests.  Tighten x86's pmd_huge() check?
    No, once it's a hugepage entry, it can get quite far from a good pmd: for
    example, PROT_NONE leaves it with only ACCESSED of the KERN_PGTABLE bits.
    
    However... though follow_page() contains this and another test for huge
    pages, so it's nice to keep it working on them, where does it actually get
    called on a huge page?  get_user_pages() checks is_vm_hugetlb_page(vma) to
    to call alternative hugetlb processing, as does unmap_vmas() and others.
    
    Signed-off-by: Hugh Dickins <hugh>
    Earlier-version-tested-by: Ingo Molnar <mingo>
    Cc: Thomas Gleixner <tglx>
    Cc: Jeff Chua <jeff.chua.linux>
    Cc: Hans Rosenfeld <hans.rosenfeld>
    Cc: Arjan van de Ven <arjan.com>
    Signed-off-by: Linus Torvalds <torvalds>