linux-kernel - Re: [PATCH RESEND v2 3/5] mm_zone: add function to check if managed dma zone exists

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Date:   Thu, 9 Dec 2021 21:23:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hch@....de, robin.murphy@....com,
        cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, vbabka@...e.cz, m.szyprowski@...sung.com,
        John.p.donnelly@...cle.com, kexec@...ts.infradead.org
Subject: Re: [PATCH RESEND v2 3/5] mm_zone: add function to check if managed
 dma zone exists

On 12/09/21 at 02:10pm, David Hildenbrand wrote:
......
> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >>> index c5952749ad40..ac0ea42a4e5f 100644
> >>> --- a/mm/page_alloc.c
> >>> +++ b/mm/page_alloc.c
> >>> @@ -9459,4 +9459,15 @@ bool take_page_off_buddy(struct page *page)
> >>>  	spin_unlock_irqrestore(&zone->lock, flags);
> >>>  	return ret;
> >>>  }
> >>> +
> >>> +bool has_managed_dma(void)
> >>> +{
> >>> +	struct zone *zone;
> >>> +
> >>> +	for_each_managed_zone(zone) {
> >>> +		if (zone_is_dma(zone))
> >>> +			return true;
> >>> +	}
> >>> +	return false;
> >>> +}
> >>
> >> Wouldn't it be "easier/faster" to just iterate online nodes and directly
> >> obtain the ZONE_DMA, checking if there are managed pages?
> > 
> > Thanks, Dave.
> > 
> > Please check for_each_managed_zone(), it is iterating online nodes and
> > it's each managed zone. 
> > 
> > Is below what you are suggesting? The only difference is I introduced
> > for_each_managed_zone() which can be reused later if needed. Not sure if
> > I got your suggestion correctly.
> > 
> > bool has_managed_dma(void)
> > {
> >         struct pglist_data *pgdat;
> >         struct zone *zone;
> >         enum zone_type i, j;
> > 
> >         for_each_online_pgdat(pgdat) {
> >                 for (i = 0; i < MAX_NR_ZONES - 1; i++) {          
> >                         struct zone *zone = &pgdat->node_zones[i];
> >                         if (zone_is_dma(zone))                                                                                                    
> >                                 return true;
> >                 }
> >         }
> >         return false;
> > 
> > }
> 
> 
> Even simpler, no need to iterate over zones at all, only over nodes:
> 
> #ifdef CONFIG_ZONE_DMA
> bool has_managed_dma(void)
> {
> 	struct pglist_data *pgdat;
> 
> 	for_each_online_pgdat(pgdat) {
> 		struct zone *zone = &pgdat->node_zones[ZONE_DMA];
> 
> 		if (managed_zone(zone)
> 			return true;
> 	}
> 	return false;
> }
> #endif /* CONFIG_ZONE_DMA */
> 
> Without CONFIG_ZONE_DMA, simply provide a dummy in the header that
> returns false.

Yeah, it only iterates the number of nodes times. I will take this in
v3. Thanks, David.