Re: [Xen-devel] [PATCH 1/2] memory: exit early from memory_exchange() upon write-back error

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/04/17 14:13, Jan Beulich wrote:
> There's no point in continuing if in the end we'll return -EFAULT
> anyway. It also seems wrong to report a chunk for which at least one
> write-back failed as successfully exchanged (albeit the indication of
> an error is also not fully correct, as the exchange happened in that
> case at least partially - retrieving the GFN to assign the memory to
> and/or handing back the information on the replacement memory didn't
> work). In any case limiting the amount of damage done to the guest
> can't be all that bad an idea.
>
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
> I'm additionally surprised we don't require input GFNs to be order
> aligned for both IN- and OUT-chunks (similarly for populate-physmap
> and decrease-reservation).

This sounds like a bug, rather than being intentional.

>
> --- a/xen/common/memory.c
> +++ b/xen/common/memory.c

As an observation, I find it amusing that there is a comment just above
this which states

/*
 * Success! Beyond this point we cannot fail for this chunk.
 */

> @@ -639,6 +639,9 @@ static long memory_exchange(XEN_GUEST_HA
>              }
>          }
>          BUG_ON( !(d->is_dying) && (j != (1UL << out_chunk_order)) );
> +
> +        if ( rc )
> +            goto fail;
>      }
>  
>      exch.nr_exchanged = exch.in.nr_extents;
>
>
>


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