linux-kernel - Re: [PATCH] mm,cma: remove pfn_range_valid

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Date:   Wed, 11 Mar 2020 09:40:52 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rik van Riel <riel@...riel.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Anshuman Khandual <anshuman.khandual@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Qian Cai <cai@....pw>, Roman Gushchin <guro@...com>
Subject: Re: [PATCH] mm,cma: remove pfn_range_valid_contig

On 3/6/20 11:06 PM, Rik van Riel wrote:
> The function pfn_range_valid_contig checks whether all memory in the
> target area is free. This causes unnecessary CMA failures, since
> alloc_contig_range will migrate movable memory out of a target range,
> and has its own sanity check early on in has_unmovable_pages, which
> is called from start_isolate_page_range & set_migrate_type_isolate.
> 
> Relying on that has_unmovable_pages call simplifies the CMA code and
> results in an increased success rate of CMA allocations.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>

Yeah, the page_count and PageHuge checks are harmful. Not sure about
PageReserved. And is anything later in the alloc_contig_range() making sure that
we are always in the same zone?

> ---
>  mm/page_alloc.c | 47 +++--------------------------------------------
>  1 file changed, 3 insertions(+), 44 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 0fb3c1719625..75e84907d8c6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8539,32 +8539,6 @@ static int __alloc_contig_pages(unsigned long start_pfn,
>  				  gfp_mask);
>  }
>  
> -static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
> -				   unsigned long nr_pages)
> -{
> -	unsigned long i, end_pfn = start_pfn + nr_pages;
> -	struct page *page;
> -
> -	for (i = start_pfn; i < end_pfn; i++) {
> -		page = pfn_to_online_page(i);
> -		if (!page)
> -			return false;
> -
> -		if (page_zone(page) != z)
> -			return false;
> -
> -		if (PageReserved(page))
> -			return false;
> -
> -		if (page_count(page) > 0)
> -			return false;
> -
> -		if (PageHuge(page))
> -			return false;
> -	}
> -	return true;
> -}
> -
>  static bool zone_spans_last_pfn(const struct zone *zone,
>  				unsigned long start_pfn, unsigned long nr_pages)
>  {
> @@ -8605,28 +8579,13 @@ struct page *alloc_contig_pages(unsigned long nr_pages, gfp_t gfp_mask,
>  	zonelist = node_zonelist(nid, gfp_mask);
>  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
>  					gfp_zone(gfp_mask), nodemask) {
> -		spin_lock_irqsave(&zone->lock, flags);
> -
>  		pfn = ALIGN(zone->zone_start_pfn, nr_pages);
>  		while (zone_spans_last_pfn(zone, pfn, nr_pages)) {
> -			if (pfn_range_valid_contig(zone, pfn, nr_pages)) {
> -				/*
> -				 * We release the zone lock here because
> -				 * alloc_contig_range() will also lock the zone
> -				 * at some point. If there's an allocation
> -				 * spinning on this lock, it may win the race
> -				 * and cause alloc_contig_range() to fail...
> -				 */
> -				spin_unlock_irqrestore(&zone->lock, flags);
> -				ret = __alloc_contig_pages(pfn, nr_pages,
> -							gfp_mask);
> -				if (!ret)
> -					return pfn_to_page(pfn);
> -				spin_lock_irqsave(&zone->lock, flags);
> -			}
> +			ret = __alloc_contig_pages(pfn, nr_pages, gfp_mask);
> +			if (!ret)
> +				return pfn_to_page(pfn);
>  			pfn += nr_pages;
>  		}
> -		spin_unlock_irqrestore(&zone->lock, flags);
>  	}
>  	return NULL;
>  }
>