Re: [PATCH v2] x86/mm: Ensure useful progress in alloc_l2_table()

[Keir Fraser <keir.xen@gmail.com>]

On 03/07/2013 13:28, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:

> While debugging the issue which turned out to be XSA-58, a printk in this loop
> showed that it was quite easy to never make useful progress, because of
> consistently failing the preemption check.
> 
> One single l2 entry is a reasonable amount of work to do, even if an action is
> pending, and also assures forwards progress across repeat continuations.
> 
> Tweak the continuation criteria to fail on the first iteration of the loop.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Keir <keir@xen.org>

Acked-by: Keir Fraser <keir@xen.org>

> Cc: Jan Beulich <JBeulich@suse.com>
> Cc: Tim Deegan <tim@xen.org>
> 
> ---
> Changes since v1:
>  * Dont move the continuation block to the end
> ---
>  xen/arch/x86/mm.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> index 77dcafc..f151dec 100644
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -1278,7 +1278,8 @@ static int alloc_l2_table(struct page_info *page,
> unsigned long type,
>  
>      for ( i = page->nr_validated_ptes; i < L2_PAGETABLE_ENTRIES; i++ )
>      {
> -        if ( preemptible && i && hypercall_preempt_check() )
> +        if ( preemptible && i > page->nr_validated_ptes
> +             && hypercall_preempt_check() )
>          {
>              page->nr_validated_ptes = i;
>              rc = -EAGAIN;