Re: [Xen-devel] [PATCH] xen/page_alloc: Keep away MFN 0 from the buddy allocator

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.08.2019 20:15, Stewart Hildebrand wrote:
> On Friday, August 9, 2019 9:39 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> > On 09.08.2019 14:14, Julien Grall wrote:
> > > Combining of buddies happens only such that the resulting larger buddy
> > > is still order-aligned. To cross a zone boundary while merging, the
> > > implication is that both the buddy [0, 2^n-1] and the buddy
> > > [2^n, 2^(n+1)] are free.
> > [2^n, 2^(n+1)-1]
> >
> > You may want to add that merging across zone boundaries is what we
> > need to prevent.
> >
> > > Ideally we want to fix the allocator, but for now we can just prevent
> > > adding the MFN 0 in the allocator.
> > >
> > > On x86, the MFN 0 is already kept away from the buddy allocator. So the
> > > bug can only happen on Arm platform where the first memory bank is
> > > starting at 0.
> > >
> > > As this is a specific to the allocator, the MFN 0 is removed in the common code
> > > to cater all the architectures (current and future).
> > >
> > > Reported-by: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Here is Jeff's initial patch for the issue.
To be honest, it would have been nice if you had clarified _why_
you sent this in reply here.

Jan

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