Re: [PATCH v2] x86/mm: fix freeing of PMD-sized vmemmap pages

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/29/26 12:49, 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().
> 
> 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.
> 
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> 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.
> ---

@x86 maintainers, do you want to take this through your tree or should we merge
this through the MM tree?

I have another MM series coming up that will touch this code (no fixes, though).

-- 
Cheers,

David