Re: [Xen-devel] [PATCH v5 3/8] mm: Scrub pages in alloc_heap_pages() if needed

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 07/23/17 4:07 AM >>>
>On 06/27/2017 02:00 PM, Jan Beulich wrote:
>>>>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 06/22/17 8:55 PM >>>
>>> @@ -862,10 +879,19 @@ static struct page_info *alloc_heap_pages(
>>>       if ( d != NULL )
>>>           d->last_alloc_node = node;
>>>   
>>> +    need_scrub = !!first_dirty_pg && !(memflags & MEMF_no_scrub);
>> 
>> No need for !! here. But I wonder whether that part of the check is really
>> useful anyway, considering the sole use ...
>> 
>>>       for ( i = 0; i < (1 << order); i++ )
>>>       {
>>>           /* Reference count must continuously be zero for free pages. */
>>> -        BUG_ON(pg[i].count_info != PGC_state_free);
>>> +        BUG_ON((pg[i].count_info & ~PGC_need_scrub) != PGC_state_free);
>>> +
>>> +        if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
>>> +        {
>>> +            if ( need_scrub )
>>> +                scrub_one_page(&pg[i]);
>> 
>> ... here. If it isn't, I think the local variable isn't warranted either.
>> If you agree, the thus adjusted patch can have
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>> (otherwise I'll wait with it to understand the reason first).
>
>first_dirty_pg is indeed unnecessary but I think local variable is 
>useful to avoid ANDing memflags inside the loop on each iteration 
>(unless you think compiler is smart enough to realize that memflags is 
>not changing).

I don't understand: At least on x86 I'd expect the compiler to use a
single TEST if you used memflags inside the loop, whereas the local
variable would likely be a single CMP inside the loop plus setup code
outside of it.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel