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

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Date:   Sun, 7 Jan 2018 10:48:21 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jike Song <albcamus@...il.com>
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, 7 Jan 2018, Jike Song wrote:
> 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.

Right, but this kind of explanation wants to be in the changelog. Empty
changelogs for this kind of change are just not acceptable.

Thanks,

	tglx