Re: [PATCH] x86/mm: Fix incorrect for loop count calculation in sync_global_pgds

From: Baoquan He <bhe@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	X86 ML <x86@kernel.org>, Kees Cook <keescook@chromium.org>,
	Thomas Garnier <thgarnie@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Yasuaki Ishimatsu <yasu.isimatu@gmail.com>,
	Jinbum Park <jinb.park7@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Yinghai Lu <yinghai@kernel.org>, Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH] x86/mm: Fix incorrect for loop count calculation in sync_global_pgds
Date: Mon, 1 May 2017 22:52:58 +0800	[thread overview]
Message-ID: <20170501145258.GI2649@x1> (raw)
In-Reply-To: <CAPcyv4jfFK3ZMGtqFRjgtB1__73Ja-94XS3=FB1Wj9SNxn+-JQ@mail.gmail.com>

On 05/01/17 at 07:40am, Dan Williams wrote:
> On Mon, May 1, 2017 at 4:41 AM, Baoquan He <bhe@redhat.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;
> > +
> 
> Let's change this to put the next address calculation in the for loop
> directly and use the ALIGN macro. Something like:
> 
>  for (address = start; address <= end; address = ALIGN(address + 1, PGDIR_SIZE))

Hi Dan,

Good idea!

Do you think below change is OK for you? Taking out the initialization
can make the for loop line be shorter than 80 char.

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 15173d3..0840311 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 = start;
 
-       for (address = start; address <= end; address += PGDIR_SIZE) {
+       for (; address <= end; address = ALIGN(address + 1, PGDIR_SIZE))
{
                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;
