Re: Page zapping and page table reclaim

[David Hildenbrand <david@redhat.com>]

On 20.03.21 02:56, Balbir Singh wrote:
> On Fri, Mar 19, 2021 at 01:44:55PM +0100, David Hildenbrand wrote:
>> On 19.03.21 00:53, Balbir Singh wrote:
>>> On Thu, Mar 18, 2021 at 05:57:06PM +0100, Vlastimil Babka wrote:
>>>> On 3/11/21 7:14 PM, David Hildenbrand wrote:
>>>>> Hi folks,
>>>>>
>>>>> I was wondering, is there any mechanism that reclaims basically empty page
>>>>> tables in a running process?
>>>>>
>>>>> Like: When I MADV_DONTNEED a huge range, there could be plenty of basically
>>>>> empty (e.g., all entries invalid) page tables we could reclaim. As soon as we
>>>>> zap a complete PMD we could reclaim (depending on the architecture) a whole page.
>>>>>
>>>>> Zapping on the PMD level might make most impact I guess.
>>>>>
>>>>> For 1 GB, we need 262144 4k pages. If we assume each PTE is 8 bytes, we need a
>>>>> total of 8 MB for the lowest level page tables (PTE).
>>>>>
>>>>> OTOH, we would need 512 PMD entries - a single 4k page. Zapping 1 TB would mean
>>>>> we can free up another 4MB - rather a corner case and we can live with that.
>>>>>
>>>>>
>>>>> Of course, the same might apply to other cases where we can restore all page
>>>>> table content from the VMA again. One example would be after MADV_FREE zapped a
>>>>> whole range of entries we marked.
>>>>
>>>> I don't think we have such mechanism, but IIRC I've heard the idea mentioned
>>>> before, probably from Michal Hocko. Definitely an interesting research project
>>>> idea to evaluate the cost vs benefits of that.
>>>>
>>>
>>> It might lead to interesting interactions with lockless page table walking
>>> with implications on the mmap_lock as well.
>>>
>>
>> I think if lockless page table walks have to be able with THP code swapping
>> populated page tables by a PMD back and forth, swapping an unpopulated page
>> table by an invalid PMD entry might be quite similar. At least it feels like
>> both approaches would rely on similar mechanisms / locking. :)
>>
> 
> Yes, but then I suspect you always need destruct page tables by RCU.

I envision page table reclaim to happen asynchronously, so that 
shouldn't be an issue. We can just collect a bunch of reclaimed page 
tables and then issue an rcu synchronize before handing them back to the 
buddy.


-- 
Thanks,

David / dhildenb