Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

David Hildenbrand
2023-02-14T17:04:08Z

On 14.02.23 17:58, David Hildenbrand wrote:
>
>>>>
>>>> Honestly, for improving the fork(), I have an idea to skip the per-page
>>>> operation without breaking the logic. However, this will introduce the
>>>> complicated mechanism and may has the overhead for other features. It
>>>> might not be worth it. It's hard to strike a balance between the
>>>> over-complicated mechanism with (probably) better performance and data
>>>> consistency with the page status. So, I would focus on the safety and
>>>> stable approach at first.
>>>
>>> Yes, it is most probably possible, but complexity, robustness and
>>> maintainability have to be considered as well.
>>>
>>> Thanks for implementing this approach (only deduplication without other
>>> optimizations) and evaluating it accordingly. It's certainly "cleaner", such
>>> that we only have to mess with unsharing and not with other
>>> accounting/pinning/mapcount thingies. But it also highlights how intrusive
>>> even this basic deduplication approach already is -- and that most benefits
>>> of the original approach requires even more complexity on top.
>>>
>>> I am not quite sure if the benefit is worth the price (I am not to decide
>>> and I would like to hear other options).
>>
>> I'm looking at the discussion of page table sharing in 2002 [1].
>> It looks like in 2002 ~ 2006, there also have some patches try to
>> improve fork().
>>
>> After that, I also saw one thread which is about another shared page
>> table patch's benchmark. I can't find the original patch though [2].
>> But, I found the probably same patch in 2005 [3], it also mentioned
>> the previous benchmark discussion:
>>
>> "
>> For those familiar with the shared page table patch I did a couple of years
>> ago, this patch does not implement copy-on-write page tables for private
>> mappings. Analysis showed the cost and complexity far outweighed any
>> potential benefit.
>> "
>
> Thanks for the pointer, interesting read. And my personal opinion is
> that part of that statement still hold true :)
>
>>
>> However, it might be different right now. For example, the implemetation
>> . We have split page table lock now, so we don't have to consider the
>> page_table_share_lock thing. Also, presently, we have different use
>> cases (shells [2] v.s. VM cloning and fuzzing) to consider.

Oh, and because I stumbled over it, just as an interesting pointer on
QEMU devel:

"[PATCH 00/10] Retire Fork-Based Fuzzing" [1]

[1] https://lore.kernel.org/all/20230205042951.3570008-1-alxndr@bu.edu/T/#u

--
Thanks,

David / dhildenb