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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 <>
> The commit...
> -----------------------------
> commit 6194ba6ff6ccf8d5c54c857600843c67aa82c407
> Author: Jeremy Fitzhardinge <>
> 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

__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 <>
> Signed-off-by: Rohit Jain <>
> Cc: Zachary Amsden <>
> Cc: Jeremy Fitzhardinge <>
> Cc:
> ---
> 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 ||
> + 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.


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