linux-kernel - Re: [PATCH v2] huge_memory: return -EINVAL in folio split functions when THP is disabled

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Message-ID: <8b646492-7615-4b46-81ec-1a1601fa7e2f@lucifer.local>
Date: Fri, 5 Sep 2025 12:57:02 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Zi Yan <ziy@...dia.com>, Ryan Roberts <ryan.roberts@....com>,
        David Hildenbrand <david@...hat.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Barry Song <baohua@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        linux-kernel@...r.kernel.org, willy@...radead.org, linux-mm@...ck.org,
        mcgrof@...nel.org, gost.dev@...sung.com,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2] huge_memory: return -EINVAL in folio split functions
 when THP is disabled

On Thu, Sep 04, 2025 at 11:51:29AM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@...sung.com>
>
> split_huge_page_to_list_[to_order](), split_huge_page() and
> try_folio_split() return 0 on success and error codes on failure.
>
> When THP is disabled, these functions return 0 indicating success even
> though an error code should be returned as it is not possible to split a
> folio when THP is disabled.
>
> Make all these functions return -EINVAL to indicate failure instead of
> 0. As large folios depend on CONFIG_THP, issue warning as this function
> should not be called without a large folio.
>
> Signed-off-by: Pankaj Raghav <p.raghav@...sung.com>

Other than the trivially-fixable issue mentioned below this LGTM so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

On basis of those being fixed.

> ---
> This issue was discovered while experimenting enabling large folios
> without THP and found that returning 0 in these functions is resulting in
> undefined behavior in truncate operations. This change fixes the issue.
>
>  include/linux/huge_mm.h | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 29ef70022da1..23f124493c47 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -588,22 +588,30 @@ static inline int
>  split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order)
>  {
> -	return 0;
> +	struct folio *folio = page_folio(page);
> +
> +	VM_WARN_ON_ONCE_FOLIO(1, folio);

As per the bot report, you should just do page_folio(page) here or otherwise
when compiled out you'll get an unused var warning.

> +	return -EINVAL;
>  }
>  static inline int split_huge_page(struct page *page)
>  {
> -	return 0;
> +	struct folio *folio = page_folio(page);

Same as above!

> +
> +	VM_WARN_ON_ONCE_FOLIO(1, folio);
> +	return -EINVAL;
>  }
>
>  static inline int split_folio_to_list(struct folio *folio, struct list_head *list)
>  {
> -	return 0;
> +	VM_WARN_ON_ONCE_FOLIO(1, folio);
> +	return -EINVAL;
>  }
>
>  static inline int try_folio_split(struct folio *folio, struct page *page,
>  		struct list_head *list)
>  {
> -	return 0;
> +	VM_WARN_ON_ONCE_FOLIO(1, folio);
> +	return -EINVAL;
>  }
>
>  static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
>
> base-commit: 291634ccfd2820c09f6e8c4982c2dee8155d09ae
> --
> 2.50.1
>