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

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Message-ID: <aJC4C7PndXlcDIro@li-2b55cdcc-350b-11b2-a85c-a78bff51fc11.ibm.com>
Date: Mon, 4 Aug 2025 15:39:23 +0200
From: Sumanth Korikkar <sumanthk@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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] mm: fix accounting of memmap pages for early sections

On Mon, Aug 04, 2025 at 02:27:20PM +0200, David Hildenbrand wrote:
> On 04.08.25 11:08, 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 also decrements the count of nr_memmap_pages.
> > 
> > * For pages from the boot allocator, free_map_bootmem() is called. But
> >    it currently does not adjust the nr_memmap_boot_pages.
> > 
> > To fix this inconsistency, update free_map_bootmem() to also decrement
> > the nr_memmap_boot_pages count by invoking memmap_boot_pages_add(),
> > mirroring how free_vmemmap_page() handles this for boot-allocated pages.
> > 
> > This ensures correct tracking of memmap pages regardless of allocation
> > source.
> > 
> > Cc: stable@...r.kernel.org
> > Fixes: 15995a352474 ("mm: report per-page metadata information")
> > Signed-off-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
> > ---
> >   mm/sparse.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/sparse.c b/mm/sparse.c
> > index 3c012cf83cc2..d7c128015397 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -688,6 +688,7 @@ static void free_map_bootmem(struct page *memmap)
> >   	unsigned long start = (unsigned long)memmap;
> >   	unsigned long end = (unsigned long)(memmap + PAGES_PER_SECTION);
> > +	memmap_boot_pages_add(-1L * (DIV_ROUND_UP(end - start, PAGE_SIZE)));
> >   	vmemmap_free(start, end, NULL);
> >   }
> 
> Looks good to me. But now I wonder about !CONFIG_SPARSEMEM_VMEMMAP, where
> neither depopulate_section_memmap() nor free_map_bootmem() adjust anything?
> 
> Which makes me wonder whether we should be moving that to
> section_deactivate().

Agree. I will move accounting to section_deactivate() then.

Thanks