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

[AndrÃs Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

Acked-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

On Dec 17, 2013, at 12:26 PM, Tim Deegan <tim@xxxxxxx> wrote:

> 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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