Re: [PATCH 1/2] mm/huge_memory: do not change split_huge_page*() target order silently.

[Lance Yang]

On 2025/10/11 01:39, Zi Yan wrote:
> Page cache folios from a file system that support large block size (LBS)
> can have minimal folio order greater than 0, thus a high order folio might
> not be able to be split down to order-0. Commit e220917fa507 ("mm: split a
> folio in minimum folio order chunks") bumps the target order of
> split_huge_page*() to the minimum allowed order when splitting a LBS folio.
> This causes confusion for some split_huge_page*() callers like memory
> failure handling code, since they expect after-split folios all have
> order-0 when split succeeds but in really get min_order_for_split() order
> folios.
>
> Fix it by failing a split if the folio cannot be split to the target order.
>
> Fixes: e220917fa507 ("mm: split a folio in minimum folio order chunks")
> [The test poisons LBS folios, which cannot be split to order-0 folios, and
> also tries to poison all memory. The non split LBS folios take more memory
> than the test anticipated, leading to OOM. The patch fixed the kernel
> warning and the test needs some change to avoid OOM.]
> Reported-by: syzbot+e6367ea2fdab6ed46056@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/68d2c943.a70a0220.1b52b.02b3.GAE@xxxxxxxxxx/
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>   include/linux/huge_mm.h | 28 +++++-----------------------
>   mm/huge_memory.c        |  9 +--------
>   mm/truncate.c           |  6 ++++--
>   3 files changed, 10 insertions(+), 33 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 8eec7a2a977b..9950cda1526a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -394,34 +394,16 @@ static inline int split_huge_page_to_list_to_order(struct page *page, struct lis
>    * Return: 0: split is successful, otherwise split failed.
>    */
>   static inline int try_folio_split(struct folio *folio, struct page *page,
> -		struct list_head *list)
> +		struct list_head *list, unsigned int order)

Seems like we need to add the order parameter to the stub for 
try_folio_split() as well?

#ifdef CONFIG_TRANSPARENT_HUGEPAGE

...

#else /* CONFIG_TRANSPARENT_HUGEPAGE */

static inline int try_folio_split(struct folio *folio, struct page *page,
		struct list_head *list)
{
	VM_WARN_ON_ONCE_FOLIO(1, folio);
	return -EINVAL;
}

#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

Cheers,
Lance