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@xxxxxx/T/#u Thanks for the information. It's interesting. Thanks, Chih-En Lin
