linux-kernel - Re: [PATCHv2] mm/migrate: clean up useless code in migrate_vma_collect

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Date:   Thu, 15 Aug 2019 15:40:21 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     Pingfan Liu <kernelfans@...il.com>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] mm/migrate: clean up useless code in
 migrate_vma_collect_pmd()

On Thu, Aug 15, 2019 at 12:23:44PM -0700, Ralph Campbell wrote:
> 
> On 8/15/19 10:19 AM, Jerome Glisse wrote:
> > On Wed, Aug 07, 2019 at 04:41:12PM +0800, Pingfan Liu wrote:
> > > Clean up useless 'pfn' variable.
> > 
> > NAK there is a bug see below:
> > 
> > > 
> > > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > > Cc: "Jérôme Glisse" <jglisse@...hat.com>
> > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Mel Gorman <mgorman@...hsingularity.net>
> > > Cc: Jan Kara <jack@...e.cz>
> > > Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > > Cc: Michal Hocko <mhocko@...e.com>
> > > Cc: Mike Kravetz <mike.kravetz@...cle.com>
> > > Cc: Andrea Arcangeli <aarcange@...hat.com>
> > > Cc: Matthew Wilcox <willy@...radead.org>
> > > To: linux-mm@...ck.org
> > > Cc: linux-kernel@...r.kernel.org
> > > ---
> > >   mm/migrate.c | 9 +++------
> > >   1 file changed, 3 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 8992741..d483a55 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -2225,17 +2225,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > >   		pte_t pte;
> > >   		pte = *ptep;
> > > -		pfn = pte_pfn(pte);
> > >   		if (pte_none(pte)) {
> > >   			mpfn = MIGRATE_PFN_MIGRATE;
> > >   			migrate->cpages++;
> > > -			pfn = 0;
> > >   			goto next;
> > >   		}
> > >   		if (!pte_present(pte)) {
> > > -			mpfn = pfn = 0;
> > > +			mpfn = 0;
> > >   			/*
> > >   			 * Only care about unaddressable device page special
> > > @@ -2252,10 +2250,10 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > >   			if (is_write_device_private_entry(entry))
> > >   				mpfn |= MIGRATE_PFN_WRITE;
> > >   		} else {
> > > +			pfn = pte_pfn(pte);
> > >   			if (is_zero_pfn(pfn)) {
> > >   				mpfn = MIGRATE_PFN_MIGRATE;
> > >   				migrate->cpages++;
> > > -				pfn = 0;
> > >   				goto next;
> > >   			}
> > >   			page = vm_normal_page(migrate->vma, addr, pte);
> > > @@ -2265,10 +2263,9 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> > >   		/* FIXME support THP */
> > >   		if (!page || !page->mapping || PageTransCompound(page)) {
> > > -			mpfn = pfn = 0;
> > > +			mpfn = 0;
> > >   			goto next;
> > >   		}
> > > -		pfn = page_to_pfn(page);
> > 
> > You can not remove that one ! Otherwise it will break the device
> > private case.
> > 
> 
> I don't understand. The only use of "pfn" I see is in the "else"
> clause above where it is set just before using it.

Ok i managed to confuse myself with mpfn and probably with old
version of the code. Sorry for reading too quickly. Can we move
unsigned long pfn; into the else { branch so that there is no
more confusion to its scope.

Cheers,
Jérôme