Re: [RFC][Patch v12 1/2] mm: page_reporting: core infrastructure

[Nitesh Narayan Lal <nitesh@redhat.com>]

On 8/15/19 9:15 AM, Nitesh Narayan Lal wrote:
> On 8/14/19 12:11 PM, Alexander Duyck wrote:
>> On Wed, Aug 14, 2019 at 8:49 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>> On 8/12/19 2:47 PM, Alexander Duyck wrote:
>>>> On Mon, Aug 12, 2019 at 6:13 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>>>>> This patch introduces the core infrastructure for free page reporting in
>>>>> virtual environments. It enables the kernel to track the free pages which
>>>>> can be reported to its hypervisor so that the hypervisor could
>>>>> free and reuse that memory as per its requirement.
>>>>>
>>>>> While the pages are getting processed in the hypervisor (e.g.,
>>>>> via MADV_DONTNEED), the guest must not use them, otherwise, data loss
>>>>> would be possible. To avoid such a situation, these pages are
>>>>> temporarily removed from the buddy. The amount of pages removed
>>>>> temporarily from the buddy is governed by the backend(virtio-balloon
>>>>> in our case).
>>>>>
>>>>> To efficiently identify free pages that can to be reported to the
>>>>> hypervisor, bitmaps in a coarse granularity are used. Only fairly big
>>>>> chunks are reported to the hypervisor - especially, to not break up THP
>>>>> in the hypervisor - "MAX_ORDER - 2" on x86, and to save space. The bits
>>>>> in the bitmap are an indication whether a page *might* be free, not a
>>>>> guarantee. A new hook after buddy merging sets the bits.
>>>>>
>>>>> Bitmaps are stored per zone, protected by the zone lock. A workqueue
>>>>> asynchronously processes the bitmaps, trying to isolate and report pages
>>>>> that are still free. The backend (virtio-balloon) is responsible for
>>>>> reporting these batched pages to the host synchronously. Once reporting/
>>>>> freeing is complete, isolated pages are returned back to the buddy.
>>>>>
>>>>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>>> [...]
>>>>> +}
>>>>> +
>>>>> +/**
>>>>> + * __page_reporting_enqueue - tracks the freed page in the respective zone's
>>>>> + * bitmap and enqueues a new page reporting job to the workqueue if possible.
>>>>> + */
>>>>> +void __page_reporting_enqueue(struct page *page)
>>>>> +{
>>>>> +       struct page_reporting_config *phconf;
>>>>> +       struct zone *zone;
>>>>> +
>>>>> +       rcu_read_lock();
>>>>> +       /*
>>>>> +        * We should not process this page if either page reporting is not
>>>>> +        * yet completely enabled or it has been disabled by the backend.
>>>>> +        */
>>>>> +       phconf = rcu_dereference(page_reporting_conf);
>>>>> +       if (!phconf)
>>>>> +               return;
>>>>> +
>>>>> +       zone = page_zone(page);
>>>>> +       bitmap_set_bit(page, zone);
>>>>> +
>>>>> +       /*
>>>>> +        * We should not enqueue a job if a previously enqueued reporting work
>>>>> +        * is in progress or we don't have enough free pages in the zone.
>>>>> +        */
>>>>> +       if (atomic_read(&zone->free_pages) >= phconf->max_pages &&
>>>>> +           !atomic_cmpxchg(&phconf->refcnt, 0, 1))
>>>> This doesn't make any sense to me. Why are you only incrementing the
>>>> refcount if it is zero? Combining this with the assignment above, this
>>>> isn't really a refcnt. It is just an oversized bitflag.
>>> The intent for having an extra variable was to ensure that at a time only one
>>> reporting job is enqueued. I do agree that for that purpose I really don't need
>>> a reference counter and I should have used something like bool
>>> 'page_hinting_active'. But with bool, I think there could be a possible chance
>>> of race. Maybe I should rename this variable and keep it as atomic.
>>> Any thoughts?
>> You could just use a bitflag to achieve what you are doing here. That
>> is the primary use case for many of the test_and_set_bit type
>> operations. However one issue with doing it as a bitflag is that you
>> have no way of telling that you took care of all requesters.
> I think you are right, I might end up missing on certain reporting
> opportunities in some special cases. Specifically when the pages which are
> part of this new reporting request belongs to a section of the bitmap which
> has already been scanned. Although, I have failed to reproduce this kind of
> situation in an actual setup.
>
>>  That is
>> where having an actual reference count comes in handy as you know
>> exactly how many zones are requesting to be reported on.
>
> True.
>
>>>> Also I am pretty sure this results in the opportunity to miss pages
>>>> because there is nothing to prevent you from possibly missing a ton of
>>>> pages you could hint on if a large number of pages are pushed out all
>>>> at once and then the system goes idle in terms of memory allocation
>>>> and freeing.
>>> I was looking at how you are enqueuing/processing reporting jobs for each zone.
>>> I am wondering if I should also consider something on similar lines as having
>>> that I might be able to address the concern which you have raised above. But it
>>> would also mean that I have to add an additional flag in the zone_flags. :)
>> You could do it either in the zone or outside the zone as yet another
>> bitmap. I decided to put the flags inside the zone because there was a
>> number of free bits there and it should be faster since we were
>> already using the zone structure.
> There are two possibilities which could happen while I am reporting:
> 1. Another request might come in for a different zone.
> 2. Another request could come in for the same zone and the pages belong to a
>     section of the bitmap which has already been scanned.
>
> Having a per zone flag to indicate reporting status will solve the first
> issue and to an extent the second as well. Having refcnt will possibly solve
> both of them. What I am wondering about is that in my case I could easily
> impact the performance negatively by performing more bitmap scanning.
>
>

I realized that it may not be possible for me to directly adopt either refcnt
or zone flags just because of the way I have page reporting setup right now.

For now, I will just replace the refcnt with a bitflag as that should work
for most of the cases.  Nevertheless, I will also keep looking for a better way.

-- 
Thanks
Nitesh