Re: [PATCH] KVM: PPC: Book3S HV: Fix refcounting of hugepages

[Avi Kivity <avi@redhat.com>]

On 05/08/2012 04:10 PM, Alexander Graf wrote:
> On 08.05.2012, at 12:24, Paul Mackerras wrote:
>
> > From: David Gibson <david@gibson.dropbear.id.au>
> > 
> > The H_REGISTER_VPA hcall implementation in HV Power KVM needs to pin some
> > guest memory pages into host memory so that they can be safely accessed
> > from usermode.  It does this used get_user_pages_fast().  When the VPA is
> > unregistered, or the VCPUs are cleaned up, these pages are released using
> > put_page().
> > 
> > However, the get_user_pages() is invoked on the specific memory are of the
> > VPA which could lie within hugepages.  In case the pinned page is huge,
> > we explicitly find the head page of the compound page before calling
> > put_page() on it.
> > 
> > At least with the latest kernel, this is not correct.  put_page() already
> > handles finding the correct head page of a compound, and also deals with
> > various counts on the individual tail page which are important for
> > transparent huge pages.  We don't support transparent hugepages on Power,
> > but even so, bypassing this count maintenance can lead (when the VM ends)
> > to a hugepage being released back to the pool with a non-zero mapcount on
> > one of the tail pages.  This can then lead to a bad_page() when the page
> > is released from the hugepage pool.
> > 
> > This removes the explicit compound_head() call to correct this bug.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > Signed-off-by: Paul Mackerras <paulus@samba.org>
>
> Acked-by: Alexander Graf <agraf@suse.de>
>
> Avi, could you please make sure this makes the next 3.4-rc or -stable?
>
>

Sure, applied to master, will push tomorrow.

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