Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

[Huang\, Ying]

David Hildenbrand <david@xxxxxxxxxx> writes:

> On 02.03.20 15:12, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxxxx> writes:
>> 
>>> On Fri 28-02-20 16:55:40, Huang, Ying wrote:
>>>> David Hildenbrand <david@xxxxxxxxxx> writes:
>>> [...]
>>>>> E.g., free page reporting in QEMU wants to use MADV_FREE. The guest will
>>>>> report currently free pages to the hypervisor, which will MADV_FREE the
>>>>> reported memory. As long as there is no memory pressure, there is no
>>>>> need to actually free the pages. Once the guest reuses such a page, it
>>>>> could happen that there is still the old page and pulling in in a fresh
>>>>> (zeroed) page can be avoided.
>>>>>
>>>>> AFAIKs, after your change, we would get more pages discarded from our
>>>>> guest, resulting in more fresh (zeroed) pages having to be pulled in
>>>>> when a guest touches a reported free page again. But OTOH, page
>>>>> migration is speed up (avoiding to migrate these pages).
>>>>
>>>> Let's look at this problem in another perspective.  To migrate the
>>>> MADV_FREE pages of the QEMU process from the node A to the node B, we
>>>> need to free the original pages in the node A, and (maybe) allocate the
>>>> same number of pages in the node B.  So the question becomes
>>>>
>>>> - we may need to allocate some pages in the node B
>>>> - these pages may be accessed by the application or not
>>>> - we should allocate all these pages in advance or allocate them lazily
>>>>   when they are accessed.
>>>>
>>>> We thought the common philosophy in Linux kernel is to allocate lazily.
>>>
>>> The common philosophy is to cache as much as possible.
>> 
>> Yes.  This is another common philosophy.  And MADV_FREE pages is
>> different from caches such as the page caches because it has no valid
>> contents.
>
> Side note: It might contain valid content until discarded/zeroed out.
> E.g., an application could use a marker bit (e.g., first bit) to detect
> if the page still contains valid data or not. If the data is still
> marked valid, the content could be reuse immediately. Not sure if there
> is any such application, though :)

I don't think this is the typical use case.  But I admit that this is
possible.

Best Regards,
Huang, Ying