Re: [Xen-devel] [PATCH 4/5] x86/mem_sharing: check for errors in p2m->set_entry().

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Mar 7, 2013, at 9:53 AM, Tim Deegan <tim@xxxxxxx> wrote:

> This call ought always to succeed.  Assert that it does rather than
> ignoring the return value.
> 
> Signed-off-by: Tim Deegan <tim@xxxxxxx>
> Cc: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> ---
> xen/arch/x86/mm/mem_sharing.c |   12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
> index 1caa900..0364bb0 100644
> --- a/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -1273,6 +1273,8 @@ int relinquish_shared_pages(struct domain *d)
>         p2m_access_t a;
>         p2m_type_t t;
>         mfn_t mfn;
> +        int set_rc;
> +
>         if ( atomic_read(&d->shr_pages) == 0 )
>             break;
>         mfn = p2m->get_entry(p2m, gfn, &t, &a, 0, NULL);
> @@ -1281,10 +1283,12 @@ int relinquish_shared_pages(struct domain *d)
>             /* Does not fail with ENOMEM given the DESTROY flag */
>             BUG_ON(__mem_sharing_unshare_page(d, gfn, 
>                     MEM_SHARING_DESTROY_GFN));
> -            /* Clear out the p2m entry so no one else may try to 
> -             * unshare */
> -            p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
> -                            p2m_invalid, p2m_access_rwx);
> +            /* Clear out the p2m entry so no one else may try to
> +             * unshare.  Must succeed: we just read the old entry and
> +             * we hold the p2m lock. */
> +            set_rc = p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
> +                                    p2m_invalid, p2m_access_rwx);
> +            ASSERT(set_rc != 0);
Acked-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

Wouldn't it be slightly cleaner to BUG_ON(p2m->set_entry(..) != 1)?

Andres
>             count++;
>         }
> 
> -- 
> 1.7.10.4
> 


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