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

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Date:   Thu, 15 Aug 2019 12:23:44 -0700
From:   Ralph Campbell <rcampbell@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        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 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.