Re: [Xen-devel] [PATCH v6 1/8] mm: Place unscrubbed pages at the end of pagelist

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 08/07/2017 10:37 AM, Jan Beulich wrote:
>>>>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 08/07/17 4:16 PM >>>
>> On 08/06/2017 01:41 PM, Jan Beulich wrote:
>>>>>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 08/04/17 7:03 PM >>>
>>>> +                /* See if any of the pages indeed need scrubbing. */
>>>> +                if ( first_dirty != INVALID_DIRTY_IDX )
>>>> +                {
>>>> +                    if ( (1U << cur_order) > first_dirty )
>>>> +                    {
>>>> +                        for ( i = first_dirty ; i < (1U << cur_order); 
>>>> i++ )
>>>> +                            if ( test_bit(_PGC_need_scrub,
>>>> +                                          &cur_head[i].count_info) )
>>>> +                            {
>>>> +                                idx = i;
>>>> +                                break;
>>>> +                            }
>>> Why again do you need to look through all the pages here, rather than
>>> simply marking the chunks as needing scrubbing simply based on first_dirty?
>>> It seems to me that I've asked this before, which is a good indication that
>>> such special behavior would better have a comment attached.
>> We want to make sure that there are in fact dirty pages in the
>> (sub-)buddy: first_dirty is only a hint there *may be* some.
> But why is that needed? Iirc you don't go to such length when splitting a
> buddy during allocation.

I felt that allocation is more time-critical and so I decided against
trying to be "neat" as far as tracking dirty pages exactly.

-boris

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