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

[David Hildenbrand]

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 :)

-- 
Thanks,

David / dhildenb