Re: [PATCH] x86/mm: fix freeing of PMD-sized vmemmap pages

Mike Rapoport
2026-04-28T20:47:11Z

On Tue, Apr 28, 2026 at 12:29:36PM +0200, David Hildenbrand (Arm) wrote:
> In commit bf9e4e30f353 ("x86/mm: use pagetable_free()"), we switched
> from freeing non-boot page tables through __free_pages() to
> pagetable_free().
>
> However, the function is also called to free vmemmap pages.
>
> Given that vmemmap pages are not page tables, already the page_ptdesc(page)
> is wrong. But worse, pagetable_free() calls
>
> __free_pages(page, compound_order(page));
>
> As vmemmap pages are not compound pages (see vmemmap_alloc_block()) --
> except for HVO, which doesn't apply here -- we will only free the first
> page when freeing a PMD-sized vmemmap page, leaking the other ones.
>
> Fix it by properly decoupling pagetable and vmemmap freeing.
> free_pagetable() no longer has to mess with SECTION_INFO, as only the
> vmemmap is marked like that in register_page_bootmem_memmap().
>
> While at it, just wire up the altmap parameter for remove_pte_table().
> Also, the indentation in remove_pmd_table() is messed up, let's fix that
> while touching it.
>
> Note that we'll try to get rid of that bootmem info handling soon. For
> now, we'll handle it similar to free_pagetable(), just avoiding the
> ifdef.
>
> Fixes: bf9e4e30f353 ("x86/mm: use pagetable_free()")
> Cc: stable@vger.kernel.org
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> Reproduced and tested with a simple VM with a virtio-mem device,
> repeatedly adding and removing memory.
>
> Found by code inspection while working on bootmem_info removal.
> ---
> arch/x86/mm/init_64.c | 43 +++++++++++++++++++++++++++----------------
> 1 file changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index df2261fa4f98..8d03e44a7fb9 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1014,7 +1014,7 @@ static void __meminit free_pagetable(struct page *page, int order)
> #ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
> enum bootmem_type type = bootmem_type(page);
>
> - if (type == SECTION_INFO || type == MIX_SECTION_INFO) {
> + if (type == MIX_SECTION_INFO) {
> while (nr_pages--)
> put_page_bootmem(page++);
> } else {
> @@ -1028,13 +1028,24 @@ static void __meminit free_pagetable(struct page *page, int order)
> }
> }
>
> -static void __meminit free_hugepage_table(struct page *page,
> +static void __meminit free_vmemmap_pages(struct page *page, unsigned int order,
> struct vmem_altmap *altmap)
> {
> - if (altmap)
> - vmem_altmap_free(altmap, PMD_SIZE / PAGE_SIZE);
> - else
> - free_pagetable(page, get_order(PMD_SIZE));
> + if (altmap) {
> + vmem_altmap_free(altmap, 1u << order);
> + } else if (PageReserved(page)) {
> + unsigned long nr_pages = 1 << order;
> +
> + if (IS_ENABLED(CONFIG_HAVE_BOOTMEM_INFO_NODE) &&
> + bootmem_type(page) == SECTION_INFO) {
> + while (nr_pages--)
> + put_page_bootmem(page++);
> + } else {
> + free_reserved_pages(page, nr_pages);
> + }
> + } else {
> + __free_pages(page, order);
> + }
> }
>
> static void __meminit free_pte_table(pte_t *pte_start, pmd_t *pmd)
> @@ -1093,7 +1104,7 @@ static void __meminit free_pud_table(pud_t *pud_start, p4d_t *p4d)
>
> static void __meminit
> remove_pte_table(pte_t *pte_start, unsigned long addr, unsigned long end,
> - bool direct)
> + bool direct, struct vmem_altmap *altmap)
> {
> unsigned long next, pages = 0;
> pte_t *pte;
> @@ -1118,7 +1129,7 @@ remove_pte_table(pte_t *pte_start, unsigned long addr, unsigned long end,
> return;
>
> if (!direct)
> - free_pagetable(pte_page(*pte), 0);
> + free_vmemmap_pages(pte_page(*pte), 0, altmap);
>
> spin_lock(&init_mm.page_table_lock);
> pte_clear(&init_mm, addr, pte);
> @@ -1153,25 +1164,25 @@ remove_pmd_table(pmd_t *pmd_start, unsigned long addr, unsigned long end,
> if (IS_ALIGNED(addr, PMD_SIZE) &&
> IS_ALIGNED(next, PMD_SIZE)) {
> if (!direct)
> - free_hugepage_table(pmd_page(*pmd),
> - altmap);
> + free_vmemmap_pages(pmd_page(*pmd),
> + PMD_ORDER, altmap);
>
> spin_lock(&init_mm.page_table_lock);
> pmd_clear(pmd);
> spin_unlock(&init_mm.page_table_lock);
> pages++;
> } else if (vmemmap_pmd_is_unused(addr, next)) {
> - free_hugepage_table(pmd_page(*pmd),
> - altmap);
> - spin_lock(&init_mm.page_table_lock);
> - pmd_clear(pmd);
> - spin_unlock(&init_mm.page_table_lock);
> + free_vmemmap_pages(pmd_page(*pmd), PMD_ORDER,
> + altmap);
> + spin_lock(&init_mm.page_table_lock);
> + pmd_clear(pmd);
> + spin_unlock(&init_mm.page_table_lock);
> }
> continue;
> }
>
> pte_base = (pte_t *)pmd_page_vaddr(*pmd);
> - remove_pte_table(pte_base, addr, next, direct);
> + remove_pte_table(pte_base, addr, next, direct, altmap);
> free_pte_table(pte_base, pmd);
> }
>
>
> ---
>
> base-commit: a2ddbfd1af0f54ea84bf17f0400088815d012e8d
>
> change-id: 20260428-vmemmap-ab4b949aa727
>
> --
>
> Cheers,
>
> David
>

--
Sincerely yours,
Mike.