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

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Message-ID: <20210723123342.26406-1-miles.chen@mediatek.com>
Date:   Fri, 23 Jul 2021 20:33:42 +0800
From:   Miles Chen <miles.chen@...iatek.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <wsd_upstream@...iatek.com>,
        Miles Chen <miles.chen@...iatek.com>
Subject: [PATCH v3] mm/sparse: clarify pgdat_to_phys

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
-- 
2.18.0