Re: [RFC PATCH v2 9/9] mm: Introduce Copy-On-Write PTE table

[Chih-En Lin]

On Tue, Sep 27, 2022 at 06:38:05PM +0000, Nadav Amit wrote:
> I only skimmed the patches that you sent. The last couple of patches seem a
> bit rough and dirty, so I am sorry to say that I skipped them (too many
> “TODO” and “XXX” for my taste).
> 
> I am sure other will have better feedback than me. I understand there is a
> tradeoff and that this mechanism is mostly for high performance
> snapshotting/forking. It would be beneficial to see whether this mechanism
> can somehow be combined with existing ones (mshare?).

Still thanks for your feedback. :)
I'm looking at the PTE refcount and mshare patches. And, maybe it can
combine with them in the future.

> The code itself can be improved. I found the reasoning about synchronization
> and TLB flushes and synchronizations to be lacking, and the code to seem
> potentially incorrect. Better comments would help, even if the code is
> correct.
> 
> There are additional general questions. For instance, when sharing a
> page-table, do you properly update the refcount/mapcount of the mapped
> pages? And are there any possible interactions with THP?

Since access to those mapped pages will cost a lot of time, and this
will make fork() even have more overhead. It will not update the
refcount/mapcount of the mapped pages.

I'm not familiar with THP right now. But we have a plan for looking
at it to see what will happen with COW PTE.
Currently, I can only say that I prefer to avoid involving the behavior
of huge-page/THP. If there are any ideas here please tell us.

Thanks,
Chih-En Lin