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

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.


>>> 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
ÂÂÂ 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.


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