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

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Message-ID: <YzNUwxU44mq+KnCm@strix-laptop>
Date:   Wed, 28 Sep 2022 03:53:39 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     Nadav Amit <namit@...are.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        William Kucharski <william.kucharski@...cle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Tong Tiangen <tongtiangen@...wei.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Li kunyu <kunyu@...china.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Minchan Kim <minchan@...nel.org>,
        Yang Shi <shy828301@...il.com>, Song Liu <song@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Dinglan Peng <peng301@...due.edu>,
        Pedro Fonseca <pfonseca@...due.edu>,
        Jim Huang <jserv@...s.ncku.edu.tw>,
        Huichun Feng <foxhoundsk.tw@...il.com>
Subject: Re: [RFC PATCH v2 9/9] mm: Introduce Copy-On-Write PTE table

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