Re: [PATCH 2/4] mm/huge_memory: add page->private check back in __split_folio_to_order()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 6/29/26 04:56, Zi Yan wrote:
> page->private should not be set in tail pages. Commit 4265d67e405a
> ("mm/migrate_device: add THP splitting during migration") removed it
> without a proper reason. Add it back.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  mm/huge_memory.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2bccb0a53a0a..037d67fbec6e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3594,6 +3594,16 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		new_folio->mapping = folio->mapping;
>  		new_folio->index = folio->index + i;
>  
> +		/*
> +		 * page->private should not be set in tail pages. Fix up and warn once
> +		 * if private is unexpectedly set. Do it before swap.val assignment
> +		 * since private overlaps with swap.val.
> +		 */
> +		if (unlikely(new_folio->private)) {
> +			VM_WARN_ON_ONCE_PAGE(true, new_head);
> +			new_folio->private = NULL;
> +		}

The unconditional warning means this is not expected to happen. In that case
it's odd to check and fixup always, but only warn with CONFIG_DEBUG_VM.

If we are reasonably sure the current code is OK, and only want to catch new
mistakes in development, we could just VM_WARN_ON_ONCE_PAGE() without fixup.

If we are paranoid, leave it as it is, but drop the "VM_" ?

> +
>  		if (folio_test_swapcache(folio))
>  			new_folio->swap.val = folio->swap.val + i;
>  
>