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

john.p.donnelly
2021-12-13T14:22:27Z

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@vger.kernel.org
> Signed-off-by: Baoquan He <bhe@redhat.com>

>
Acked-by: John Donnelly <john.p.donnelly@oracle.com>
Tested-by: John Donnelly <john.p.donnelly@oracle.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 */