Re: [PATCH] mm/compaction: Remove some useless code in compact_zone()

[Vlastimil Babka]

On 10/14/20 2:28 PM, David Hildenbrand wrote:
> On 14.10.20 09:23, yanfei.xu@xxxxxxxxxxxxx wrote:
> > From: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> >
> > start_pfn has been declared at the begin of compact_zone(), it's
> > no need to declare it again. And remove an useless semicolon.
> >
> > Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> > ---
> >  mm/compaction.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 176dcded298e..5e69c1f94d96 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;
> > +		start_pfn = cc->migrate_pfn;
>
> There is a user in
>
> trace_mm_compaction_end(start_pfn, cc->migrate_pfn, cc->free_pfn,
> end_pfn, sync, ret);
>
> we would now trace a different value, no?

Agreed. We should rather rename the while-local one to avoid confusion. 
Something like "iteration_start_pfn" ?

> >  
> >  		/*
> >  		 * Avoid multiple rescans which can happen if a page cannot be
> > @@ -2309,7 +2309,6 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
> >  		case ISOLATE_SUCCESS:
> >  			update_cached = false;
> >  			last_migrated_pfn = start_pfn;
> > -			;
>
> Huh, how does something like that happen :)

Looks like "case ISOLATE_SUCCESS:" used to be an empty implementation, then 
statements got added, but semicolon not removed.