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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 9 Aug 2019, 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.

I committed Julien's patch for now, but if we need to make any changes
or decide for a better alternative, we can always revert it.

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