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

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Message-ID: <0a9bc228-21ba-abe3-d9c8-b9d52b936366@redhat.com>
Date:   Tue, 7 Dec 2021 12:23:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org
Cc:     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 07.12.21 04:07, Baoquan He wrote:
> In some places of the current kernel, it assumes that dma zone must have
> managed pages if CONFIG_ZONE_DMA is enabled. While this is not always true.
> E.g in kdump kernel of x86_64, only low 1M is presented and locked down
> at very early stage of boot, so that there's no managed pages at all in
> DMA zone. This exception will always cause page allocation failure if page
> is requested from DMA zone.
> 
> Here add function has_managed_dma() and the relevant helper functions to
> check if there's DMA zone with managed pages. It will be used in later
> patches.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  include/linux/mmzone.h | 21 +++++++++++++++++++++
>  mm/page_alloc.c        | 11 +++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 58e744b78c2c..82d23e13e0e5 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -998,6 +998,18 @@ static inline bool zone_is_zone_device(struct zone *zone)
>  }
>  #endif
>  
> +#ifdef CONFIG_ZONE_DMA
> +static inline bool zone_is_dma(struct zone *zone)
> +{
> +	return zone_idx(zone) == ZONE_DMA;
> +}
> +#else
> +static inline bool zone_is_dma(struct zone *zone)
> +{
> +	return false;
> +}
> +#endif
> +
>  /*
>   * Returns true if a zone has pages managed by the buddy allocator.
>   * All the reclaim decisions have to use this function rather than
> @@ -1046,6 +1058,7 @@ static inline int is_highmem_idx(enum zone_type idx)
>  #endif
>  }
>  
> +bool has_managed_dma(void);
>  /**
>   * is_highmem - helper function to quickly check if a struct zone is a
>   *              highmem zone or not.  This is an attempt to keep references
> @@ -1131,6 +1144,14 @@ extern struct zone *next_zone(struct zone *zone);
>  			; /* do nothing */		\
>  		else
>  
> +#define for_each_managed_zone(zone)		        \
> +	for (zone = (first_online_pgdat())->node_zones; \
> +	     zone;					\
> +	     zone = next_zone(zone))			\
> +		if (!managed_zone(zone))		\
> +			; /* do nothing */		\
> +		else
> +
>  static inline struct zone *zonelist_zone(struct zoneref *zoneref)
>  {
>  	return zoneref->zone;
> 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,

David / dhildenb