linux-kernel - Re: [PATCH v3 3/5] mm_zone: add function to check if managed dma zone exists

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Message-ID: <ab387012-c1a8-c3f0-8c84-dadf82ce2d9e@oracle.com>
Date:   Mon, 13 Dec 2021 08:22:13 -0600
From:   john.p.donnelly@...cle.com
To:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org, hch@....de,
        cl@...ux.com, John.p.donnelly@...cle.com,
        kexec@...ts.infradead.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 3/5] mm_zone: add function to check if managed dma zone
 exists

On 12/13/21 6:27 AM, 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.
> 
> Fixes: 6f599d84231f ("x86/kdump: Always reserve the low 1M when the crashkernel option is specified")
> Cc: stable@...r.kernel.org
> Signed-off-by: Baoquan He <bhe@...hat.com>

 >
  Acked-by: John Donnelly <john.p.donnelly@...cle.com>
  Tested-by:  John Donnelly <john.p.donnelly@...cle.com>

> ---
> v2->v3:
>   Rewrite has_managed_dma() in a simpler and more efficient way which is
>   sugggested by DavidH.
> 
>   include/linux/mmzone.h |  9 +++++++++
>   mm/page_alloc.c        | 15 +++++++++++++++
>   2 files changed, 24 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 58e744b78c2c..6e1b726e9adf 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1046,6 +1046,15 @@ static inline int is_highmem_idx(enum zone_type idx)
>   #endif
>   }
>   
> +#ifdef CONFIG_ZONE_DMA
> +bool has_managed_dma(void);
> +#else
> +static inline bool has_managed_dma(void)
> +{
> +	return false;
> +}
> +#endif
> +
>   /**
>    * is_highmem - helper function to quickly check if a struct zone is a
>    *              highmem zone or not.  This is an attempt to keep references
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c5952749ad40..7c7a0b5de2ff 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -9460,3 +9460,18 @@ bool take_page_off_buddy(struct page *page)
>   	return ret;
>   }
>   #endif
> +
> +#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 */