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Subject: Re: [PATCH 2/2] mm/page_alloc: Allow high-order pages to be stored on
 the per-cpu lists
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
References: <20210603084621.24109-1-mgorman@techsingularity.net>
 <20210603084621.24109-3-mgorman@techsingularity.net>
 <1c15b083-26f8-4473-80e6-bcc2f549ba41@suse.cz>
 <20210603123437.GW30378@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <7657abef-cc50-b6a1-3b6a-697d97a3d4e2@suse.cz>
Date:   Thu, 3 Jun 2021 15:04:50 +0200
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On 6/3/21 2:34 PM, Mel Gorman wrote:
> 
>> > @@ -687,10 +687,53 @@ static void bad_page(struct page *page, const char *reason)
>> >  	add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
>> >  }
>> >  
>> > +static inline unsigned int order_to_pindex(int migratetype, int order)
>> > +{
>> > +	int base = order;
>> > +
>> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> > +	if (order > PAGE_ALLOC_COSTLY_ORDER) {
>> > +		VM_BUG_ON(order != pageblock_order);
>> > +		base = PAGE_ALLOC_COSTLY_ORDER + 1;
>> > +	}
>> > +#else
>> > +	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>> > +#endif
>> > +
>> > +	return (MIGRATE_PCPTYPES * base) + migratetype;
>> > +}
>> > +
>> > +static inline int pindex_to_order(unsigned int pindex)
>> > +{
>> > +	int order = pindex / PAGE_ALLOC_COSTLY_ORDER;
>> 
>> This seems wrong, shouldn't we divide by MIGRATE_PCPTYPES?
>> It just happens to be the same number, so testing won't flag this.
>> 
> 
> Whoops, yes, basic algebra says you're right so well spotted. If
> PAGE_ALLOC_COSTLY_ORDER ever changed for some reason, this would blow up.
> 
> So this?

Yeah, great!

> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index bbe05289b121..f1bed5b847ec 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -345,11 +345,11 @@ enum zone_watermarks {
>  #define NR_PCP_LISTS (MIGRATE_PCPTYPES * (PAGE_ALLOC_COSTLY_ORDER + 1 + NR_PCP_THP))
>  
>  /*
> - * Shift to encode migratetype in order in the least significant bits and
> - * migratetype in the higher bits.
> + * Shift to encode migratetype and order in the same integer, with order
> + * in the least significant bits.
>   */
> -#define NR_PCP_ORDER_SHIFT 8
> -#define NR_PCP_ORDER_MASK ((1<<NR_PCP_ORDER_SHIFT) - 1)
> +#define NR_PCP_ORDER_WIDTH 8
> +#define NR_PCP_ORDER_MASK ((1<<NR_PCP_ORDER_WIDTH) - 1)
>  
>  #define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost)
>  #define low_wmark_pages(z) (z->_watermark[WMARK_LOW] + z->watermark_boost)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 49f3c7dcdfca..7be71f9ad9f0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -694,7 +694,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>  
>  static inline int pindex_to_order(unsigned int pindex)
>  {
> -	int order = pindex / PAGE_ALLOC_COSTLY_ORDER;
> +	int order = pindex / MIGRATE_PCPTYPES;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> @@ -1487,7 +1487,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  			batch_free = count;
>  
>  		order = pindex_to_order(pindex);
> -		BUILD_BUG_ON(MAX_ORDER >= (1<<NR_PCP_ORDER_SHIFT));
> +		BUILD_BUG_ON(MAX_ORDER >= (1<<NR_PCP_ORDER_WIDTH));
>  		do {
>  			page = list_last_entry(list, struct page, lru);
>  			/* must delete to avoid corrupting pcp list */
> @@ -1499,7 +1499,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  				continue;
>  
>  			/* Encode order with the migratetype */
> -			page->index <<= NR_PCP_ORDER_SHIFT;
> +			page->index <<= NR_PCP_ORDER_WIDTH;
>  			page->index |= order;
>  
>  			list_add_tail(&page->lru, &head);
> @@ -1537,7 +1537,7 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  
>  		/* mt has been encoded with the order (see above) */
>  		order = mt & NR_PCP_ORDER_MASK;
> -		mt >>= NR_PCP_ORDER_SHIFT;
> +		mt >>= NR_PCP_ORDER_WIDTH;
>  
>  		/* MIGRATE_ISOLATE page should not go to pcplists */
>  		VM_BUG_ON_PAGE(is_migrate_isolate(mt), page);
> 
> 
>