Re: [PATCHv2] mm/migrate: clean up useless code in migrate_vma_collect_pmd()

[Ralph Campbell]

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@xxxxxxxxx>
> > Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > To: linux-mm@xxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >   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.