Re: [Xen-devel] [PATCH v2] x86/memshr: fix preemption in relinquish_shared_pages()

[Tim Deegan <tim@xxxxxxx>]

At 15:23 +0000 on 17 Dec (1387290199), Jan Beulich wrote:
> For one, should hypercall_preempt_check() return false the first time
> it gets called, it would never have got called again (because count,
> being checked for equality, didn't get reset to zero).
> 
> And then, if there were a huge range of unshared pages, with count not
> getting incremented at all in that case there would also not be any
> preemption.
> 
> Fix this by using a biased increment (ratio 1:16 for unshared vs shared
> pages), and flushing the count to zero in case of a "false" return from
> hypercall_preempt_check().
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Tim Deegan <tim@xxxxxxx>

> ---
> v2: Adjust continuation detection as per Tim's request.
> 
> --- b/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -1290,15 +1290,21 @@ int relinquish_shared_pages(struct domai
>              set_rc = p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
>                                      p2m_invalid, p2m_access_rwx);
>              ASSERT(set_rc != 0);
> -            count++;
> +            count += 0x10;
>          }
> +        else
> +            ++count;
>  
> -        /* Preempt every 2MiB. Arbitrary */
> -        if ( (count == 512) && hypercall_preempt_check() )
> +        /* Preempt every 2MiB (shared) or 32MiB (unshared) - arbitrary. */
> +        if ( count >= 0x2000 )
>          {
> -            p2m->next_shared_gfn_to_relinquish = gfn + 1;
> -            rc = -EAGAIN;
> -            break;
> +            if ( hypercall_preempt_check() )
> +            {
> +                p2m->next_shared_gfn_to_relinquish = gfn + 1;
> +                rc = -EAGAIN;
> +                break;
> +            }
> +            count = 0;
>          }
>      }
>  


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