Re: [Xen-devel] [PATCH] xen/p2m: Don't call get_balloon_scratch_page() twice, keep interrupts disabled for multicalls

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 6 Sep 2013, Boris Ostrovsky wrote:
> m2p_remove_override() calls get_balloon_scratch_page() in
> MULTI_update_va_mapping() even though it already has pointer to this page from
> the earlier call (in scratch_page). This second call doesn't have a matching
> put_balloon_scratch_page() thus not restoring preempt count back. (Also, there
> is no put_balloon_scratch_page() in the error path.)
> 
> In addition, the second multicall uses __xen_mc_entry() which does not disable
> interrupts. Rearrange xen_mc_* calls to keep interrupts off while performing
> multicalls.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>


>  arch/x86/xen/p2m.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 0d4ec35..8b901e8 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -990,10 +990,13 @@ int m2p_remove_override(struct page *page,
>                               printk(KERN_WARNING "m2p_remove_override: "
>                                               "pfn %lx mfn %lx, failed to 
> modify kernel mappings",
>                                               pfn, mfn);
> +                             put_balloon_scratch_page();
>                               return -1;
>                       }
>  
> -                     mcs = xen_mc_entry(
> +                     xen_mc_batch();
> +
> +                     mcs = __xen_mc_entry(
>                                       sizeof(struct 
> gnttab_unmap_and_replace));
>                       unmap_op = mcs.args;
>                       unmap_op->host_addr = kmap_op->host_addr;
> @@ -1003,12 +1006,11 @@ int m2p_remove_override(struct page *page,
>                       MULTI_grant_table_op(mcs.mc,
>                                       GNTTABOP_unmap_and_replace, unmap_op, 
> 1);
>  
> -                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> -
>                       mcs = __xen_mc_entry(0);
>                       MULTI_update_va_mapping(mcs.mc, scratch_page_address,
> -                                     
> pfn_pte(page_to_pfn(get_balloon_scratch_page()),
> +                                     pfn_pte(page_to_pfn(scratch_page),
>                                       PAGE_KERNEL_RO), 0);
> +
>                       xen_mc_issue(PARAVIRT_LAZY_MMU);
>  
>                       kmap_op->host_addr = 0;
> -- 
> 1.8.3.1
> 

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