[PATCH v2] mm/mm_init: Don't iterate pages below ARCH_PFN_OFFSET

[Ruihan Li <lrh2000@pku.edu.cn>]

Currently, memmap_init initializes pfn_hole with 0 instead of
ARCH_PFN_OFFSET. Then init_unavailable_range will start iterating each
page from the page at address zero to the first available page, but it
won't do anything for pages below ARCH_PFN_OFFSET because pfn_valid
won't pass.

If ARCH_PFN_OFFSET is very large (e.g., something like 2^64-2GiB if the
kernel is used as a library and loaded at a very high address), the
pointless iteration for pages below ARCH_PFN_OFFSET will take a very
long time, and the kernel will look stuck at boot time.

This commit sets the initial value of pfn_hole to ARCH_PFN_OFFSET, which
avoids the problematic and useless iteration mentioned above.

This problem has existed since commit 907ec5fca3dc ("mm: zero remaining
unavailable struct pages").

Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
---
Link to v1:
 - https://lore.kernel.org/linux-mm/20250418162727.1535335-1-lrh2000@pku.edu.cn/
Changes since v1:
 - Removed the unnecessary Fixes tag.
 - Fixed the build issue for CONFIG_SPARSEMEM.

 mm/mm_init.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 84f14fa12..a697a83ff 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -969,6 +969,15 @@ static void __init memmap_init(void)
 	unsigned long hole_pfn = 0;
 	int i, j, zone_id = 0, nid;
 
+#ifdef CONFIG_FLATMEM
+	/*
+	 * Pages below ARCH_PFN_OFFSET are invalid as far as pfn_valid is
+	 * concerned, so don't waste time iterating on them when looking
+	 * for holes.
+	 */
+	hole_pfn = ARCH_PFN_OFFSET;
+#endif
+
 	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
 		struct pglist_data *node = NODE_DATA(nid);
 
-- 
2.49.0