Re: [PATCH v3 3/3] mm/kmemleak: stop the per-cpu and struct page scans early too

Re: [PATCH v3 3/3] mm/kmemleak: stop the per-cpu and struct page scans early too

[Breno Leitao]

Hello Oleg,

On Tue, Jun 16, 2026 at 12:07:04PM +0200, Oleg Nesterov wrote:
> On 06/15, Breno Leitao wrote:
> >
> >  #ifdef CONFIG_SMP
> >  	/* per-cpu sections scanning */
> > -	for_each_possible_cpu(i)
> > -		scan_large_block(__per_cpu_start + per_cpu_offset(i),
> > -				 __per_cpu_end + per_cpu_offset(i));
> > +	for_each_possible_cpu(i) {
> > +		if (scan_large_block(__per_cpu_start + per_cpu_offset(i),
> > +				     __per_cpu_end + per_cpu_offset(i)))
> > +			break;
> > +	}
> >  #endif
> 
> The patch looks correct, but...
> 
> if scan_large_block() returns true, then
> 
> > @@ -1902,6 +1908,7 @@ static void kmemleak_scan(void)
> >  		unsigned long start_pfn = zone->zone_start_pfn;
> >  		unsigned long end_pfn = zone_end_pfn(zone);
> >  		unsigned long pfn;
> > +		int stop = 0;
> >
> >  		for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> >  			struct page *page = pfn_to_online_page(pfn);
> > @@ -1918,8 +1925,12 @@ static void kmemleak_scan(void)
> >  			/* only scan if page is in use */
> >  			if (page_count(page) == 0)
> >  				continue;
> > -			scan_block(page, page + 1, NULL);
> > +			stop = scan_block(page, page + 1, NULL);
> 
> it is pointless to enter this for_each_populated_zone() loop and call
> kmemleak_scan_task_stacks() after that?

Upon further review, your semll is right, once a phase trips scan_should_stop()
the later phases are still entered and each just bails on its first
scan_block(). That can be optimized away, e.g. by lifting 'stop' to function
scope and guarding the remaining phases so they aren't entered at all.

It is not a regression, though -- the pointless work is pre-existing.

Before this series none of these loops broke out on scan_should_stop(): the
scan ground through every CPU, every pfn and every thread, relying only on
scan_block() returning early on each call. This series already removes the bulk
of that; the bit you spotted (entering the next phase only to bail) is the
small leftover.

So I'd rather keep this series as-is and do that optimization as a follow-up
once it lands, instead of growing a patch that's already been reviewed. Are
you OK with that?

Thanks,
--breno

Re: [PATCH v3 3/3] mm/kmemleak: stop the per-cpu and struct page scans early too

[Oleg Nesterov]

Hi Breno,

On 06/16, Breno Leitao wrote:
>
> So I'd rather keep this series as-is and do that optimization as a follow-up
> once it lands, instead of growing a patch that's already been reviewed. Are
> you OK with that?

OK, agreed.

I don't think you need my ack, but FWIW

Reviewed-by: Oleg Nesterov <oleg@redhat.com>