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Mon, 19 Oct 2020 11:39:50 +0000 Received: from BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::adbd:559a:4a78:f09b]) by BY5PR11MB4241.namprd11.prod.outlook.com ([fe80::adbd:559a:4a78:f09b%6]) with mapi id 15.20.3477.028; Mon, 19 Oct 2020 11:39:49 +0000 Subject: Re: [PATCH v2] mm/compaction: Rename 'start_pfn' to 'iteration_start_pfn' in compact_zone() To: Vlastimil Babka , 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> <0480a649-9f49-90fe-fbf2-be1d2df306f0@suse.cz> From: "Xu, Yanfei" Message-ID: Date: Mon, 19 Oct 2020 19:39:41 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 In-Reply-To: <0480a649-9f49-90fe-fbf2-be1d2df306f0@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [60.247.85.82] X-ClientProxiedBy: HK0PR01CA0049.apcprd01.prod.exchangelabs.com (2603:1096:203:a6::13) To BY5PR11MB4241.namprd11.prod.outlook.com (2603:10b6:a03:1ca::13) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [128.224.162.160] (60.247.85.82) by HK0PR01CA0049.apcprd01.prod.exchangelabs.com (2603:1096:203:a6::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3477.21 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-OriginatorOrg: windriver.com X-MS-Exchange-CrossTenant-Network-Message-Id: 067a3580-682d-4458-41fd-08d87423afbb X-MS-Exchange-CrossTenant-AuthSource: BY5PR11MB4241.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Oct 2020 11:39:49.6086 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ddb2873-a1ad-4a18-ae4e-4644631433be X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: csJYdv3pT1ibt+HQGxW4+vpcFekIWqgK5xXA/GMFRsAF2CavTRJF7EM9jGVIGv9KuDPeWUaLmtwcXdyG9OFZxw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR11MB2839 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>>> >>>> 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 >>>> Acked-by: Vlastimil Babka >>>> Signed-off-by: Yanfei Xu >>>> --- >>>> 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, >>>> >>> >>> >> >