Re: [PATCH v2] mm: fix accounting of memmap pages for early sections

[Wei Yang <richard.weiyang@gmail.com>]

On Mon, Aug 04, 2025 at 05:13:27PM +0200, Sumanth Korikkar wrote:
>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@vger.kernel.org
>Fixes: 15995a352474 ("mm: report per-page metadata information")
>Suggested-by: David Hildenbrand <david@redhat.com>
>Signed-off-by: Sumanth Korikkar <sumanthk@linux.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);
>+	}

The change here is reasonable. While maybe we still miss the counting at some
other points.

For example:

a. 

  sparse_init_nid()
    __populate_section_memmap()

If !CONFIG_SPARSEMEM_VMEMMAP, and sparse_buffer_alloc() return NULL, it
allocate extra memory from bootmem, which looks not counted.

b. 

  section_activate()
    populate_section_memmap()

If !CONFIG_SPARSEMEM_VMEMMAP, it just call kvmalloc_node(), which looks not
counted.

Do I missed something?

> 
> 	if (empty)
> 		ms->section_mem_map = (unsigned long)NULL;
>-- 
>2.48.1
>

-- 
Wei Yang
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