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

[Jan Beulich <jbeulich@xxxxxxxx>]

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>

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel