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

[Wei Yang <richard.weiyang@gmail.com>]

On Fri, Oct 10, 2025 at 01:39:05PM -0400, 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@syzkaller.appspotmail.com
>Closes: https://lore.kernel.org/all/68d2c943.a70a0220.1b52b.02b3.GAE@google.com/
>Signed-off-by: Zi Yan <ziy@nvidia.com>
>---
> 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.
>  */

It is better to update the document of try_folio_split()

> static inline int try_folio_split(struct folio *folio, struct page *page,
>-		struct list_head *list)
>+		struct list_head *list, unsigned int order)
> {
>-	int ret = min_order_for_split(folio);
>-
>-	if (ret < 0)
>-		return ret;
>-
>-	if (!non_uniform_split_supported(folio, 0, false))
>+	if (!non_uniform_split_supported(folio, order, false))
> 		return split_huge_page_to_list_to_order(&folio->page, list,
>-				ret);
>-	return folio_split(folio, ret, page, list);
>+				order);
>+	return folio_split(folio, order, page, list);
> }

-- 
Wei Yang
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