Messages in this thread | | | Date | Thu, 05 Feb 2009 18:30:34 -0800 | From | Jeremy Fitzhardinge <> | Subject | Re: [PARAVIRT/x86] BUGFIX: Put a missing paravirt_release_pmd in pgd_dtor |
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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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