[PATCH v2] mm/sparse: clarify pgdat_to_phys

[Miles Chen]

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://lore.kernel.org/linux-arm-kernel/20210615131902.GB47121@C02TD0UTHF1T.local/
[2] https://lore.kernel.org/patchwork/patch/1452903/#1650759

Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>

---

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");
+#endif /* !CONFIG_NUMA */
 	return __pa(pgdat);
-#endif
 }
 
 #ifdef CONFIG_MEMORY_HOTREMOVE
-- 
2.18.0