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    SubjectRe: [PARAVIRT/x86] BUGFIX: Put a missing paravirt_release_pmd in pgd_dtor
    Alok Kataria wrote:
    > [PARAVIRT/x86] Put a missing paravirt_release_pmd in pgd_dtor.
    >
    > From: Alok N Kataria <akataria@vmware.com>
    >
    > The commit...
    > -----------------------------
    > commit 6194ba6ff6ccf8d5c54c857600843c67aa82c407
    > Author: Jeremy Fitzhardinge <jeremy@goop.org>
    > Date: Wed Jan 30 13:34:11 2008 +0100
    >
    > x86: don't special-case pmd allocations as much
    > ------------------------------
    > ...made changes to the way we handle pmd allocations, and while doing that
    > it (accidently ?) dropped a call to paravirt_release_pd from the pgd_dtor
    > code path.
    >

    Well, any fallout to VMI was accidental - but I'm surprised it took you
    over a year to notice a problem here...

    > As a result of this missing release, the hypervisor is now unaware of the
    > pgd page being freed, and as a result it ends up tracking this page as a
    > page table page.
    > After this the guest may start using the same page for other purposes, and
    > depending on what use the page is put to, it may result in various performance
    > and/or functional issues ( hangs, reboots).
    >

    In Xen we're critically dependent on getting calls to
    paravirt_release_pmd right, with rather immediate and bad results if its
    wrong. So I suspect something else is up here.

    (I assume we're talking 32-bit PAE here, since VMI is 32-bit only and
    the pmd only comes into play with PAE.)

    Right now, paravirt_release_pmd is being called from __pmd_free_tlb and
    pgd_mop_up_pmds.

    __pmd_free_tlb is called on all the free paths of mappings being removed
    via munmap or exit.
    pgd_mop_up_pmds is called to clear out any opportunistically allocated
    pmds which were never used for process-created mappings.

    In either case, it will be called before the page is freed back to the
    kernel's heap and reused.

    > The patch below adds a paravirt_release_pmd call for the PGD page.
    > Patch on top of 2.6.29-rc3 (mainline-git).
    >
    > Signed-off-by: Alok N Kataria <akataria@vmware.com>
    > Signed-off-by: Rohit Jain <rjain@vmware.com>
    > Cc: Zachary Amsden <zach@vmware.com>
    > Cc: Jeremy Fitzhardinge <jeremy@goop.org>
    > Cc: stable@kernel.org
    > ---
    >
    > arch/x86/mm/pgtable.c | 6 ++++++
    > 1 files changed, 6 insertions(+), 0 deletions(-)
    >
    >
    > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
    > index 86f2ffc..c23cd7e 100644
    > --- a/arch/x86/mm/pgtable.c
    > +++ b/arch/x86/mm/pgtable.c
    > @@ -89,6 +89,12 @@ static void pgd_dtor(pgd_t *pgd)
    > {
    > unsigned long flags; /* can be called from interrupt context */
    >
    > + if (PAGETABLE_LEVELS == 2 ||
    > + (PAGETABLE_LEVELS == 3 && SHARED_KERNEL_PMD) ||
    > + PAGETABLE_LEVELS == 4) {
    > + paravirt_release_pmd(__pa(pgd) >> PAGE_SHIFT);
    > + }
    >

    Ah, you want release_pmd to be called on pgds as well... Why? Do you
    track the page type for pgds? Why not just make a copy of the entries
    on cr3 reload like the real hardware does?

    Alternatively you could hook pv_mmu_ops.exit_mmap to get a call when the
    last reference to the pagetable has been dropped.

    Or, if you really must, introduce paravirt_release_pgd and hook that.

    But either way, calling release_pmd here is wrong, since its only meant
    to be applied to pmds, and it would break the Xen code.

    J


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