linux-kernel - Re: [PATCH] x86/mm: Fix incorrect for loop count calculation in sync_global

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Date:   Tue, 2 May 2017 09:24:49 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     Yinghai Lu <yinghai@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        Jinbum Park <jinb.park7@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Young <dyoung@...hat.com>
Subject: Re: [PATCH] x86/mm: Fix incorrect for loop count calculation in
 sync_global_pgds


* Baoquan He <bhe@...hat.com> wrote:

> On 05/01/17 at 03:37pm, Yinghai Lu wrote:
> > On Mon, May 1, 2017 at 4:41 AM, Baoquan He <bhe@...hat.com> wrote:
> > >  arch/x86/mm/init_64.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> > > index 15173d3..dbf4f00 100644
> > > --- a/arch/x86/mm/init_64.c
> > > +++ b/arch/x86/mm/init_64.c
> > > @@ -94,12 +94,14 @@ __setup("noexec32=", nonx32_setup);
> > >   */
> > >  void sync_global_pgds(unsigned long start, unsigned long end)
> > >  {
> > > -       unsigned long address;
> > > +       unsigned long address, address_next;
> > >
> > > -       for (address = start; address <= end; address += PGDIR_SIZE) {
> > > +       for (address = start; address <= end; address = address_next) {
> > >                 const pgd_t *pgd_ref = pgd_offset_k(address);
> > >                 struct page *page;
> > >
> > > +               address_next = (address & PGDIR_MASK) + PGDIR_SIZE;
> > > +
> > >                 if (pgd_none(*pgd_ref))
> > >                         continue;
> > >
> > 
> > This one is better than V2.
> > 
> > It would better if could rename address to addr as Ingo suggested.
> 
> Thanks for your checking and suggestion, Yinghai.
> 
> Both v1 and v2 are fine to me. As you said, code in v1 is easily
> understood, while v2 code is more compact, less line. The line of v1 is
> not more than 80. Maybe Ingo can help choose one which he likes better.

Let's do the variant I suggested - that makes the loop self-contained ('continue' 
would work as-is, etc.) and makes the code all around more readable.

Thanks,

	Ingo