linux-kernel - Re: [PATCH] x86/mm/pti: remove dead logic during user pagetable population

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Message-ID: <CANE52Kh9aCNoW-QTaqPZ-MrwKAn_gJ=wFbhMxAN6ZFVLci5ZVQ@mail.gmail.com>
Date:   Sun, 7 Jan 2018 11:00:59 +0800
From:   Jike Song <albcamus@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     luto@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/mm/pti: remove dead logic during user pagetable population

On Sun, Jan 7, 2018 at 3:33 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Sun, 7 Jan 2018, Jike Song wrote:
>
> Care to explain why you think this is not needed?
>

Hi Thomas,

Look at one of the original code snippets:

    162         if (pgd_none(*pgd)) {
    163                 unsigned long new_p4d_page = __get_free_page(gfp);
    164                 if (!new_p4d_page)
    165                         return NULL;
    166
    167                 if (pgd_none(*pgd)) {
    168                         set_pgd(pgd, __pgd(_KERNPG_TABLE |
__pa(new_p4d_page)));
    169                         new_p4d_page = 0;
    170                 }
    171                 if (new_p4d_page)
    172                         free_page(new_p4d_page);
    173         }

Correct me if I'm too dumb to see the rationale here, but to me there
can't be any difference between
two pgd_none(*pgd) of L162 and L167, so it is always false in L171.


> Thanks,
>
>         tglx


-- 
Thanks,
Jike