linux-kernel - Re: [RFC PATCH v2 2/9] mm: pgtable: Add sysctl to enable COW PTE

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Date:   Wed, 28 Sep 2022 16:36:15 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     John Hubbard <jhubbard@...dia.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-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>, Nadav Amit <namit@...are.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 2/9] mm: pgtable: Add sysctl to enable COW PTE

On Tue, Sep 27, 2022 at 02:22:27PM -0700, John Hubbard wrote:
> > +extern int sysctl_cow_pte_pid;
>
> So are setting a global value, to a single pid?? Only one pid at a time
> can be set up?
>
> I think that tells you already that there is a huge API problem here.
>
> As the other thread with Nadav said, this is not a sysctl. It wants to
> be a prctl(), at least the way things look so far.

I will change it to use the prctl().
Probably it will be under PR_SET_MM or create a new option.

> > +static void set_cow_pte_task(void)
> > +{
> > +   struct pid *pid;
> > +   struct task_struct *task;
> > +
> > +   pid = find_get_pid(sysctl_cow_pte_pid);
>
> This seems to be missing a corresponding call to put_pid().

Thanks.

> thanks,
>
> --
> John Hubbard
> NVIDIA
>

Best regards,
Chih-En Lin