Re: [PATCH] mm: sparsemem: fix missing higher order allocation splitting

[Mike Kravetz <mike.kravetz@oracle.com>]

On 06/20/22 10:30, Muchun Song wrote:
> Higher order allocations for vmemmap pages from buddy allocator must be
> able to be treated as indepdenent small pages as they can be freed
> individually by the caller.  There is no problem for higher order vmemmap
> pages allocated at boot time since each individual small page will be
> initialized at boot time.  However, it will be an issue for memory hotplug
> case since those higher order vmemmap pages are allocated from buddy
> allocator without initializing each individual small page's refcount. The
> system will panic in put_page_testzero() when CONFIG_DEBUG_VM is enabled
> if the vmemmap page is freed.
> 
> Fixes: d8d55f5616cf ("mm: sparsemem: use page table lock to protect kernel pmd operations")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/sparse-vmemmap.c | 8 ++++++++
>  1 file changed, 8 insertions(+)

Thanks for fixing,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz

> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 652f11a05749..ebb489fcf07c 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -78,6 +78,14 @@ static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
>  
>  	spin_lock(&init_mm.page_table_lock);
>  	if (likely(pmd_leaf(*pmd))) {
> +		/*
> +		 * Higher order allocations from buddy allocator must be able to
> +		 * be treated as indepdenent small pages (as they can be freed
> +		 * individually).
> +		 */
> +		if (!PageReserved(page))
> +			split_page(page, get_order(PMD_SIZE));
> +
>  		/* Make pte visible before pmd. See comment in pmd_install(). */
>  		smp_wmb();
>  		pmd_populate_kernel(&init_mm, pmd, pgtable);
> -- 
> 2.11.0
>