linux-kernel - [PATCH v2] mm: fix accounting of memmap pages for early sections

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Message-ID: <20250804151328.2326642-1-sumanthk@linux.ibm.com>
Date: Mon,  4 Aug 2025 17:13:27 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>
Cc: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>, sumanthk@...ux.ibm.com
Subject: [PATCH v2] mm: fix accounting of memmap pages for early sections

memmap pages can be allocated either from the memblock (boot) allocator
during early boot or from the buddy allocator.

When these memmap pages are removed via arch_remove_memory(), the
deallocation path depends on their source:

* For pages from the buddy allocator, depopulate_section_memmap() is
  called, which should decrement the count of nr_memmap_pages.

* For pages from the boot allocator, free_map_bootmem() is called, which
  should decrement the count of the nr_memmap_boot_pages.

Ensure correct tracking of memmap pages for both early sections and non
early sections by adjusting the accounting in section_deactivate().

Cc: stable@...r.kernel.org
Fixes: 15995a352474 ("mm: report per-page metadata information")
Suggested-by: David Hildenbrand <david@...hat.com>
Signed-off-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
---
v2: consider accounting for !CONFIG_SPARSEMEM_VMEMMAP.

 mm/sparse.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 3c012cf83cc2..b9cc9e548f80 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -680,7 +680,6 @@ static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,
 	unsigned long start = (unsigned long) pfn_to_page(pfn);
 	unsigned long end = start + nr_pages * sizeof(struct page);
 
-	memmap_pages_add(-1L * (DIV_ROUND_UP(end - start, PAGE_SIZE)));
 	vmemmap_free(start, end, altmap);
 }
 static void free_map_bootmem(struct page *memmap)
@@ -856,10 +855,14 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 	 * The memmap of early sections is always fully populated. See
 	 * section_activate() and pfn_valid() .
 	 */
-	if (!section_is_early)
+	if (!section_is_early) {
+		memmap_pages_add(-1L * (DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)));
 		depopulate_section_memmap(pfn, nr_pages, altmap);
-	else if (memmap)
+	} else if (memmap) {
+		memmap_boot_pages_add(-1L * (DIV_ROUND_UP(nr_pages * sizeof(struct page),
+				      PAGE_SIZE)));
 		free_map_bootmem(memmap);
+	}
 
 	if (empty)
 		ms->section_mem_map = (unsigned long)NULL;
-- 
2.48.1