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

Chih-En Lin
2023-02-14T17:56:45Z

On Tue, Feb 14, 2023 at 06:03:58PM +0100, David Hildenbrand wrote:
> 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 for the information.
It's interesting.

Thanks,
Chih-En Lin