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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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Date:   Mon, 19 Oct 2020 19:39:41 +0800
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On 10/19/20 6:48 PM, Vlastimil Babka wrote:
> On 10/19/20 12:29 PM, Xu, Yanfei wrote:
>>
>>
>> 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?
> 
> compact_zone()
> {
>      unsigned long start_pfn = cc->zone->zone_start_pfn;
> 
>      while ((ret = compact_finished(cc)) == COMPACT_CONTINUE) {
>          unsigned long start_pfn = cc->migrate_pfn;
>      ...
>      }
> 
>      trace_mm_compaction_end(start_pfn, cc->migrate_pfn, ...)
> }
> 
> Unless my C knowledge fails me completely, the start_pfn in the while 
> loop is a new different local variable that shadows the start_pfn from 
> compact_zone() level, but does not modify its value. After while loop 
> finishes, start_pfn has still the value assigned at
> compact_zone() beginning and that's what tracepoint sees.
> 
You are right! and I got your point. As you said, the second start_pfn 
is out of scope at that point.
Will send v3.

Many thanks,
Yanfei


> So renaming the variable in while loop is not a bug fix, but removing 
> confusion.
> 
>> 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,
>>>>
>>>
>>>
>>
>