linux-kernel - Re: [PATCH v3] mm/sparse: clarify pgdat_to

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Date:   Fri, 23 Jul 2021 14:54:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miles Chen <miles.chen@...iatek.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, wsd_upstream@...iatek.com
Subject: Re: [PATCH v3] mm/sparse: clarify pgdat_to_phys

On 23.07.21 14:33, Miles Chen wrote:
> Clarify pgdat_to_phys() by testing if
> pgdat == &contig_page_data when CONFIG_NUMA=n.
> 
> We only expect contig_page_data in such case, so we
> use &contig_page_data directly instead of pgdat.
> 
> No functional change intended when CONFIG_BUG_VM=n.
> 
> Comment from Mark [1]:
> "
> ... and I reckon it'd be clearer and more robust to define
> pgdat_to_phys() in the same ifdefs as contig_page_data so
> that these, stay in-sync. e.g. have:
> 
> | #ifdef CONFIG_NUMA
> | #define pgdat_to_phys(x)	virt_to_phys(x)
> | #else /* CONFIG_NUMA */
> |
> | extern struct pglist_data contig_page_data;
> | ...
> | #define pgdat_to_phys(x)	__pa_symbol(&contig_page_data)
> |
> | #endif /* CONIFIG_NUMA */
> "
> 
> [1] https://lore.kernel.org/linux-arm-kernel/20210615131902.GB47121@C02TD0UTHF1T.local/
> 
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Miles Chen <miles.chen@...iatek.com>
> 
> --
> 
> Change since v1:
> Thanks for Mike's comment, check if pgdat == &contig_page_data,
> so it is clearer that we only expect contig_page_data when
> CONFIG_NUMA=n.
> 
> Change since v2:
> use VM_BUG_ON() to avoid runtime checking when CONFIG_BUG_VM=n
> ---
>   mm/sparse.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 6326cdf36c4f..d13d831f88a5 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -348,7 +348,8 @@ size_t mem_section_usage_size(void)
>   static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
>   {
>   #ifndef CONFIG_NUMA
> -	return __pa_symbol(pgdat);
> +	VM_BUG_ON(pgdat != &contig_page_data);
> +	return __pa_symbol(&contig_page_data);
>   #else
>   	return __pa(pgdat);
>   #endif
> 

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb