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To:     "Xu, Yanfei" <yanfei.xu@windriver.com>, 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>
 <57730b8a-f5d7-d6c7-3961-3fa95701aba5@suse.cz>
 <4aa294a0-c256-7e89-55af-6a7c790eec4d@windriver.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v2] mm/compaction: Rename 'start_pfn' to
 'iteration_start_pfn' in compact_zone()
Message-ID: <0480a649-9f49-90fe-fbf2-be1d2df306f0@suse.cz>
Date:   Mon, 19 Oct 2020 12:48:28 +0200
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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.

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,
>>>
>> 
>> 
>