linux-kernel - Re: [RESEND PATCH v2] mm/sparse: clarify pgdat_to

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Message-ID: <1627029602.14347.6.camel@mtkswgap22>
Date:   Fri, 23 Jul 2021 16:40:02 +0800
From:   Miles Chen <miles.chen@...iatek.com>
To:     David Hildenbrand <david@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <wsd_upstream@...iatek.com>
Subject: Re: [RESEND PATCH v2] mm/sparse: clarify pgdat_to_phys

On Fri, 2021-07-23 at 09:10 +0200, David Hildenbrand wrote:
> On 23.07.21 09:01, Miles Chen wrote:
> > Clarify pgdat_to_phys() by testing if
> > pgdat == &contig_page_data when CONFIG_NUMA=n.
> > 
> > contig_page_data is only available when CONFIG_NUMA=n
> > so we have to use #ifndef here.
> > 
> > No functional change intended.
> > 
> > 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 */
> > "
> > 
> > Comment from Mike [2]:
> > "
> > I'm not sure a macro is better than a static inline.
> > 
> > Maybe we'd want to warn if pgdat passed to pgtat_to_phys() is not
> > &contig_page_data, e.g something like
> > 
> > static inline phys_addr_t pgdat_to_phys(struct pglist_data *pgdat)
> > {
> > 	if (!IS_ENABLED(CONFIG_NUMA)) {
> > 		if (pgdat == &contig_page_data)
> > 			return __pa_symbol(&contig_page_data);
> > 		else
> > 			pr_warn("Unexpected pglist_data pointer!\n");
> > 	}
> > 
> > 	return __pa(pgdat);
> > }
> > "
> > 
> > [1] https://urldefense.com/v3/__https://lore.kernel.org/linux-arm-kernel/20210615131902.GB47121@C02TD0UTHF1T.local/__;!!CTRNKA9wMg0ARbw!1bJTnH2gJ6QvsBKTXH8xOiKfHF-xq_1dok3cnETD4e4qjryntFm8K0XjtEmkHAxG6g$ 
> > [2] https://urldefense.com/v3/__https://lore.kernel.org/patchwork/patch/1452903/*1650759__;Iw!!CTRNKA9wMg0ARbw!1bJTnH2gJ6QvsBKTXH8xOiKfHF-xq_1dok3cnETD4e4qjryntFm8K0XjtEmbGXEkVg$ 
> > 
> > Cc: Mike Rapoport <rppt@...nel.org>
> > Cc: Mark Rutland <mark.rutland@....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.
> > ---
> >   mm/sparse.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/sparse.c b/mm/sparse.c
> > index 6326cdf36c4f..f73ff3c124c5 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -348,10 +348,11 @@ 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);
> > -#else
> > +	if (pgdat == &contig_page_data)
> > +		return __pa_symbol(&contig_page_data);
> > +	pr_warn("Unexpected pglist_data pointer!\n");
> 
> Shouldn't this rather be a VM_BUG_ON()?
> 
> Because it looks like something that should barely happen and we might 
> not want to perform runtime checks on each and every system?

thanks for your comment, I will use VM_BUG_ON() to check if pgdat ==
contig_page_data to avoid the runtime check.
> 
>