Re: [PATCH v2] mm/compaction: Rename 'start_pfn' to 'iteration_start_pfn' in compact_zone()
From: Xu, Yanfei
Date: Mon Oct 19 2020 - 07:39:55 EST
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@xxxxxxxxxxxxx wrote:
From: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
---
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,