Messages in this thread Patch in this message | | | From | Vlastimil Babka <> | Subject | Re: [PATCH v2] mm/compaction: Rename 'start_pfn' to 'iteration_start_pfn' in compact_zone() | 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, >>> >> >> >
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