linux-kernel - Re: [PATCH 1/2] mm/huge_memory: do not change split_huge

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Message-ID: <DF7765C3-B485-440C-8035-6BBA5A7FDA4B@nvidia.com>
Date: Mon, 13 Oct 2025 13:06:54 -0400
From: Zi Yan <ziy@...dia.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, syzkaller-bugs@...glegroups.com,
 mcgrof@...nel.org, nao.horiguchi@...il.com,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, kernel@...kajraghav.com,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, jane.chu@...cle.com,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 linux-fsdevel@...r.kernel.org, david@...hat.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, linmiaohe@...wei.com,
 syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/huge_memory: do not change split_huge_page*()
 target order silently.

On 10 Oct 2025, at 22:25, Lance Yang wrote:

> 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@...kaller.appspotmail.com
>> Closes: https://lore.kernel.org/all/68d2c943.a70a0220.1b52b.02b3.GAE@google.com/
>> Signed-off-by: Zi Yan <ziy@...dia.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.
>>    */
>>   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 */

Thanks. It is also reported by lkp robot. Will fix it.

--
Best Regards,
Yan, Zi