[PATCH v4] mm: fix accounting of memmap pages

[PATCH v4] mm: fix accounting of memmap pages

[Sumanth Korikkar]

For !CONFIG_SPARSEMEM_VMEMMAP, memmap page accounting is currently done
upfront in sparse_buffer_init(). However, sparse_buffer_alloc() may
return NULL in failure scenario.

Also, memmap pages may be allocated either from the memblock allocator
during early boot or from the buddy allocator. When removed via
arch_remove_memory(), accounting of memmap pages must reflect the
original allocation source.

To ensure correctness:
* Account memmap pages after successful allocation in sparse_init_nid()
  and section_activate().
* Account memmap pages in section_deactivate() based on allocation
  source.

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>
---
v4:
* Add fixes and suggested-by.

v3: 
* Account memmap pages for !CONFIG_SPARSEMEM_VMEMMAP and only when memmap
  allocation succeeds. Thanks Wei Yang.

v2: 
* Account memmap pages for  !CONFIG_SPARSEMEM_VMEMMAP in
  section_deactivate().  Thanks David.
* https://lore.kernel.org/all/20250804151328.2326642-1-sumanthk@linux.ibm.com/

v1: 
* Account memmap pages for early sections.
* https://lore.kernel.org/all/20250804084015.270570-1-sumanthk@linux.ibm.com/

 mm/sparse-vmemmap.c |  5 -----
 mm/sparse.c         | 15 +++++++++------
 2 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index fd2ab5118e13..41aa0493eb03 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -578,11 +578,6 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
 	if (r < 0)
 		return NULL;
 
-	if (system_state == SYSTEM_BOOTING)
-		memmap_boot_pages_add(DIV_ROUND_UP(end - start, PAGE_SIZE));
-	else
-		memmap_pages_add(DIV_ROUND_UP(end - start, PAGE_SIZE));
-
 	return pfn_to_page(pfn);
 }
 
diff --git a/mm/sparse.c b/mm/sparse.c
index 066cbf82acb8..24323122f6cb 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -454,9 +454,6 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
 	 */
 	sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
 	sparsemap_buf_end = sparsemap_buf + size;
-#ifndef CONFIG_SPARSEMEM_VMEMMAP
-	memmap_boot_pages_add(DIV_ROUND_UP(size, PAGE_SIZE));
-#endif
 }
 
 static void __init sparse_buffer_fini(void)
@@ -567,6 +564,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
 				sparse_buffer_fini();
 				goto failed;
 			}
+			memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
+							   PAGE_SIZE));
 			sparse_init_early_section(nid, map, pnum, 0);
 		}
 	}
@@ -680,7 +679,6 @@ 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 +854,14 @@ 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;
@@ -904,6 +906,7 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
 		section_deactivate(pfn, nr_pages, altmap);
 		return ERR_PTR(-ENOMEM);
 	}
+	memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));
 
 	return memmap;
 }
-- 
2.48.1

Re: [PATCH v4] mm: fix accounting of memmap pages

[Wei Yang]

On Thu, Aug 07, 2025 at 08:35:45PM +0200, Sumanth Korikkar wrote:
>For !CONFIG_SPARSEMEM_VMEMMAP, memmap page accounting is currently done
>upfront in sparse_buffer_init(). However, sparse_buffer_alloc() may
>return NULL in failure scenario.
>
>Also, memmap pages may be allocated either from the memblock allocator
>during early boot or from the buddy allocator. When removed via
>arch_remove_memory(), accounting of memmap pages must reflect the
>original allocation source.
>
>To ensure correctness:
>* Account memmap pages after successful allocation in sparse_init_nid()
>  and section_activate().
>* Account memmap pages in section_deactivate() based on allocation
>  source.
>
>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>

LGTM

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

-- 
Wei Yang
Help you, Help me

Re: [PATCH v4] mm: fix accounting of memmap pages

[David Hildenbrand]

On 07.08.25 20:35, Sumanth Korikkar wrote:
> For !CONFIG_SPARSEMEM_VMEMMAP, memmap page accounting is currently done
> upfront in sparse_buffer_init(). However, sparse_buffer_alloc() may
> return NULL in failure scenario.
> 
> Also, memmap pages may be allocated either from the memblock allocator
> during early boot or from the buddy allocator. When removed via
> arch_remove_memory(), accounting of memmap pages must reflect the
> original allocation source.
> 
> To ensure correctness:
> * Account memmap pages after successful allocation in sparse_init_nid()
>    and section_activate().
> * Account memmap pages in section_deactivate() based on allocation
>    source.
> 
> 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>
> ---
> v4:
> * Add fixes and suggested-by.
> 
> v3:
> * Account memmap pages for !CONFIG_SPARSEMEM_VMEMMAP and only when memmap
>    allocation succeeds. Thanks Wei Yang.
> 
> v2:
> * Account memmap pages for  !CONFIG_SPARSEMEM_VMEMMAP in
>    section_deactivate().  Thanks David.
> * https://lore.kernel.org/all/20250804151328.2326642-1-sumanthk@linux.ibm.com/
> 
> v1:
> * Account memmap pages for early sections.
> * https://lore.kernel.org/all/20250804084015.270570-1-sumanthk@linux.ibm.com/
> 
>   mm/sparse-vmemmap.c |  5 -----
>   mm/sparse.c         | 15 +++++++++------
>   2 files changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index fd2ab5118e13..41aa0493eb03 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -578,11 +578,6 @@ struct page * __meminit __populate_section_memmap(unsigned long pfn,
>   	if (r < 0)
>   		return NULL;
>   
> -	if (system_state == SYSTEM_BOOTING)
> -		memmap_boot_pages_add(DIV_ROUND_UP(end - start, PAGE_SIZE));
> -	else
> -		memmap_pages_add(DIV_ROUND_UP(end - start, PAGE_SIZE));
> -
>   	return pfn_to_page(pfn);
>   }
>   
> diff --git a/mm/sparse.c b/mm/sparse.c
> index 066cbf82acb8..24323122f6cb 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -454,9 +454,6 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
>   	 */
>   	sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
>   	sparsemap_buf_end = sparsemap_buf + size;
> -#ifndef CONFIG_SPARSEMEM_VMEMMAP
> -	memmap_boot_pages_add(DIV_ROUND_UP(size, PAGE_SIZE));
> -#endif
>   }
>   
>   static void __init sparse_buffer_fini(void)
> @@ -567,6 +564,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
>   				sparse_buffer_fini();
>   				goto failed;
>   			}
> +			memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
> +							   PAGE_SIZE));

IIRC, we can have partially populated boot sections, where only some 
subsections actually have a memmap ... so this calculation is possibly 
wrong in some cases.

-- 
Cheers,

David / dhildenb

Re: [PATCH v4] mm: fix accounting of memmap pages

[Andrew Morton]

On Fri, 8 Aug 2025 16:23:52 +0200 David Hildenbrand <david@redhat.com> wrote:

> > @@ -567,6 +564,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
> >   				sparse_buffer_fini();
> >   				goto failed;
> >   			}
> > +			memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
> > +							   PAGE_SIZE));
> 
> IIRC, we can have partially populated boot sections, where only some 
> subsections actually have a memmap ... so this calculation is possibly 
> wrong in some cases.

The patch (which has cc:stable!) has been in state "stuck" for 12 days
due to "possibly wrong".  Could someone please find a way to get it
unstuck?

Re: [PATCH v4] mm: fix accounting of memmap pages

[Sumanth Korikkar]

> > diff --git a/mm/sparse.c b/mm/sparse.c
> > index 066cbf82acb8..24323122f6cb 100644
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -454,9 +454,6 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
> >   	 */
> >   	sparsemap_buf = memmap_alloc(size, section_map_size(), addr, nid, true);
> >   	sparsemap_buf_end = sparsemap_buf + size;
> > -#ifndef CONFIG_SPARSEMEM_VMEMMAP
> > -	memmap_boot_pages_add(DIV_ROUND_UP(size, PAGE_SIZE));
> > -#endif
> >   }
> >   static void __init sparse_buffer_fini(void)
> > @@ -567,6 +564,8 @@ static void __init sparse_init_nid(int nid, unsigned long pnum_begin,
> >   				sparse_buffer_fini();
> >   				goto failed;
> >   			}
> > +			memmap_boot_pages_add(DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
> > +							   PAGE_SIZE));
> 
> IIRC, we can have partially populated boot sections, where only some
> subsections actually have a memmap ... so this calculation is possibly wrong
> in some cases.

In section_activate():

/*
 * The early init code does not consider partially populated initial
 * sections, it simply assumes that memory will never be referenced.  If
 * we hot-add memory into such a section then we do not need to populate
 * the memmap and can simply reuse what is already there.
 */
if (nr_pages < PAGES_PER_SECTION && early_section(ms))
        return pfn_to_page(pfn);

The patch ignores the accounting here, based on the comments
described above for partially populated initial sections.

memmap = populate_section_memmap(pfn, nr_pages, nid, altmap, pgmap);
if (!memmap) {
        section_deactivate(pfn, nr_pages, altmap);
        return ERR_PTR(-ENOMEM);
}
memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));

only bookkeeping for newly allocated memmap is performed.

Also before this patch, __populate_section_memmap() did memmap
accounting for !NULL usecases. This patch also does similar change,
but covers memmap accounting for both CONFIG_SPARSEMEM_VMEMMAP +
!CONFIG_SPARSEMEM_VMEMMAP usecases and memmap accounting based on
allocation resource. 

Let me know, if this sounds right.

Thank you.