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

[Lance Yang <lance.yang@linux.dev>]

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>
>---
>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.
>---

Cool! I just reproduced the leak with QEMU pc-dimm memory hotplug as
well.

Without the fix, nr_free_pages kept dropping after the hotplugged memory
was removed again. With the fix applied, it stays stable over repeated
add/remove cycles :)

Tested-by: Lance Yang <lance.yang@linux.dev>