Re: [PATCH v15 4/7] mm: Introduce Reported pages

[Nitesh Narayan Lal]

On 12/16/19 11:28 AM, Alexander Duyck wrote:
> On Mon, 2019-12-16 at 05:17 -0500, Nitesh Narayan Lal wrote:
>> On 12/5/19 11:22 AM, Alexander Duyck wrote:
>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> In order to pave the way for free page reporting in virtualized
>>> environments we will need a way to get pages out of the free lists and
>>> identify those pages after they have been returned. To accomplish this,
>>> this patch adds the concept of a Reported Buddy, which is essentially
>>> meant to just be the Uptodate flag used in conjunction with the Buddy
>>> page type.
>> [...]
>>
>>> +enum {
>>> +	PAGE_REPORTING_IDLE = 0,
>>> +	PAGE_REPORTING_REQUESTED,
>>> +	PAGE_REPORTING_ACTIVE
>>> +};
>>> +
>>> +/* request page reporting */
>>> +static void
>>> +__page_reporting_request(struct page_reporting_dev_info *prdev)
>>> +{
>>> +	unsigned int state;
>>> +
>>> +	/* Check to see if we are in desired state */
>>> +	state = atomic_read(&prdev->state);
>>> +	if (state == PAGE_REPORTING_REQUESTED)
>>> +		return;
>>> +
>>> +	/*
>>> +	 *  If reporting is already active there is nothing we need to do.
>>> +	 *  Test against 0 as that represents PAGE_REPORTING_IDLE.
>>> +	 */
>>> +	state = atomic_xchg(&prdev->state, PAGE_REPORTING_REQUESTED);
>>> +	if (state != PAGE_REPORTING_IDLE)
>>> +		return;
>>> +
>>> +	/*
>>> +	 * Delay the start of work to allow a sizable queue to build. For
>>> +	 * now we are limiting this to running no more than once every
>>> +	 * couple of seconds.
>>> +	 */
>>> +	schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
>>> +}
>>> +
>> I think you recently switched to using an atomic variable for maintaining page
>> reporting status as I was doing in v12.
>> Which is good, as we will not have a disagreement on it now.
> There is still some differences between our approaches if I am not
> mistaken. Specifically I have code in place so that any requests to report
> while we are actively working on reporting will trigger another pass being
> scheduled after we completed. I still believe you were lacking any logic
> like that as I recall.
>

Yes, I was specifically referring to the atomic state variable.
Though I am wondering if having an atomic variable to track page reporting state
is better than having a page reporting specific unsigned long flag, which we can
manipulate via __set_bit() and __clear_bit().

>> On a side note, apologies for not getting involved actively in the
>> discussions/reviews as I am on PTO.
> No problem. I've been mostly looking for input from the core MM
> maintainers anyway as we sorted most of the virtualization pieces some
> time ago.
>
-- 
Thanks
Nitesh