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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 9 Aug 2019, 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.
> 
> 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>

I could reproduce the problem and I confirm that this patch fixes it:

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Tested-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>



> ---
> Cc: Jarvis.Roach@xxxxxxxxxxxxxxx
> Cc: Stewart.Hildebrand@xxxxxxxxxxxxxxx
> 
>     I am not sure I fully understood the exact problem when MFN 0 is
>     given to the allocator. Feel free to suggest a better explanation.
> 
>     Can anyone able to reproduce the bug test where the patch
>     effectively solve the crash?
> ---
>  xen/common/page_alloc.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> index 453b303b5b..42b8a8ce23 100644
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -1759,6 +1759,18 @@ static void init_heap_pages(
>      bool idle_scrub = false;
>  
>      /*
> +     * Keep MFN 0 away from the buddy allocator to avoid crossing zone
> +     * boundary when merging two buddies.
> +     */
> +    if ( !mfn_x(page_to_mfn(pg)) )
> +    {
> +        if ( nr_pages-- <= 1 )
> +            return;
> +        pg++;
> +    }
> +
> +
> +    /*
>       * Some pages may not go through the boot allocator (e.g reserved
>       * memory at boot but released just after --- kernel, initramfs,
>       * etc.).
> -- 
> 2.11.0
> 

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