linux-kernel - [PATCH] mm/mm_init: Don't iterate pages below ARCH_PFN

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Message-ID: <20250418162727.1535335-1-lrh2000@pku.edu.cn>
Date: Sat, 19 Apr 2025 00:27:27 +0800
From: Ruihan Li <lrh2000@....edu.cn>
To: Mike Rapoport <rppt@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
	Ruihan Li <lrh2000@....edu.cn>
Subject: [PATCH] mm/mm_init: Don't iterate pages below ARCH_PFN_OFFSET

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.

Fixes: 907ec5fca3dc ("mm: zero remaining unavailable struct pages")
Signed-off-by: Ruihan Li <lrh2000@....edu.cn>
---
 mm/mm_init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 84f14fa12..b3ae9f797 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -966,7 +966,7 @@ static void __init memmap_init_zone_range(struct zone *zone,
 static void __init memmap_init(void)
 {
 	unsigned long start_pfn, end_pfn;
-	unsigned long hole_pfn = 0;
+	unsigned long hole_pfn = ARCH_PFN_OFFSET;
 	int i, j, zone_id = 0, nid;

 	for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, &nid) {
-- 
2.49.0