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

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Message-ID: <C4CA4A1E-5553-422E-B8C0-8A5FF17DFCED@vmware.com>
Date:   Thu, 29 Sep 2022 18:40:36 +0000
From:   Nadav Amit <namit@...are.com>
To:     Chih-En Lin <shiyn.lin@...il.com>
CC:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.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>,
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        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>,
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        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 Sep 29, 2022, at 11:29 AM, Chih-En Lin <shiyn.lin@...il.com> wrote:

> That case could be caught in copy_pte_range(): in case we'd have to allocate
>> a page via page_copy_prealloc(), we'd have to fall back to the ordinary
>> "separate page table for the child" way of doing things.
>> 
>> But that looks doable to me.
> 
> Sounds good. :)

Chih-En, I admit I did not fully read the entire correspondence and got deep
into all the details.

I would note, however, that there are several additional components that I
did not see (and perhaps missed) in your patches. Basically, there are many
page-table manipulations that are done not through the page-fault handler or
reclamation mechanisms. I did not see any of them being addressed.

So if/when you send a new version, please have a look at mprotect(),
madvise(), soft-dirty, userfaultfd and THP. In these cases, I presume, you
would have to COW-break (aka COW-unshare) the page-tables.