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Subject: Re: [PATCH v2] mm/compaction: Rename 'start_pfn' to
 'iteration_start_pfn' in compact_zone()
To:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        david@redhat.com
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20201019083632.25417-1-yanfei.xu@windriver.com>
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From:   "Xu, Yanfei" <yanfei.xu@windriver.com>
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On 10/19/20 5:40 PM, Vlastimil Babka wrote:
> On 10/19/20 10:36 AM, yanfei.xu@windriver.com wrote:
>> From: Yanfei Xu <yanfei.xu@windriver.com>
>>
>> There are two 'start_pfn' declared in compact_zone() which have
>> different meaning. Rename the second one to 'iteration_start_pfn'
>> to prevent trace_mm_compaction_end() from tracing an undesirable
>> value.
> 
> "to prevent confusion.", because trace_mm_compaction_end() has the
> correct value even before the patch - the second start_pfn is out
> of scope at that point.
> 
> Thanks
>
In the while-statement, the second start_pfn is always be reassigned the 
value of cc->migrate_pfn in every loop, also the cc->migrate_pfn might 
be changed in the loop. Does trace_mm_compaction_end() really want to 
trace the new assinged start_pfn?

Without the patch： 566e54e11（mm, compaction: remove last_migrated_pfn 
from compact_control）, there is only one start_pfn which has a fixed 
value. The trace_mm_compaction_end() trace it too.

Thus, I think the tracepoint might get an undesireble value.:)

Thanks,
Yanfei

>> BTW, remove an useless semicolon.
>>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
>> ---
>> v1->v2:
>> Rename 'start_pfn' to 'iteration_start_pfn' and change commit messages.
>>
>>   mm/compaction.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 176dcded298e..ccd27c739fd6 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -2272,7 +2272,7 @@ compact_zone(struct compact_control *cc, struct 
>> capture_control *capc)
>>       while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
>>           int err;
>> -        unsigned long start_pfn = cc->migrate_pfn;
>> +        unsigned long iteration_start_pfn = cc->migrate_pfn;
>>           /*
>>            * Avoid multiple rescans which can happen if a page cannot be
>> @@ -2284,7 +2284,7 @@ compact_zone(struct compact_control *cc, struct 
>> capture_control *capc)
>>            */
>>           cc->rescan = false;
>>           if (pageblock_start_pfn(last_migrated_pfn) ==
>> -            pageblock_start_pfn(start_pfn)) {
>> +            pageblock_start_pfn(iteration_start_pfn)) {
>>               cc->rescan = true;
>>           }
>> @@ -2308,8 +2308,7 @@ compact_zone(struct compact_control *cc, struct 
>> capture_control *capc)
>>               goto check_drain;
>>           case ISOLATE_SUCCESS:
>>               update_cached = false;
>> -            last_migrated_pfn = start_pfn;
>> -            ;
>> +            last_migrated_pfn = iteration_start_pfn;
>>           }
>>           err = migrate_pages(&cc->migratepages, compaction_alloc,
>>
> 
>