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

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Date:   Thu, 15 Aug 2019 13:19:18 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     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 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.