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

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Message-ID: <34d34fac-9a8b-4496-a6db-725c40d0408b@redhat.com>
Date: Mon, 4 Aug 2025 17:24:36 +0200
From: David Hildenbrand <david@...hat.com>
To: Sumanth Korikkar <sumanthk@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
 LKML <linux-kernel@...r.kernel.org>
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>
Subject: Re: [PATCH v2] mm: fix accounting of memmap pages for early sections

On 04.08.25 17:13, 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@...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;

Acked-by: David Hildenbrand <david@...hat.com>

Hopefully we're not missing anything important.

-- 
Cheers,

David / dhildenb