linux-kernel - Re: [PATCH v2 1/3] mm/huge_memory: do not change split_huge

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Message-ID: <49BBF89F-C185-4991-B0BB-7CE7AC8130EA@nvidia.com>
Date: Thu, 16 Oct 2025 10:32:17 -0400
From: Zi Yan <ziy@...dia.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linmiaohe@...wei.com, david@...hat.com, jane.chu@...cle.com,
 kernel@...kajraghav.com,
 syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com, akpm@...ux-foundation.org,
 mcgrof@...nel.org, nao.horiguchi@...il.com,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 1/3] mm/huge_memory: do not change split_huge_page*()
 target order silently.

On 16 Oct 2025, at 3:31, Wei Yang wrote:

> On Wed, Oct 15, 2025 at 11:34:50PM -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.
>> Rename try_folio_split() to try_folio_split_to_order() to reflect the added
>> new_order parameter. Remove its unused list parameter.
>>
>> 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>
>> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
>> Reviewed-by: Pankaj Raghav <p.raghav@...sung.com>
>
> Do we want to cc stable?

This only triggers a warning, so I am inclined not to.
But some config decides to crash on kernel warnings. If anyone thinks
it is worth ccing stable, please let me know.

>
>> ---
>> include/linux/huge_mm.h | 55 +++++++++++++++++------------------------
>> mm/huge_memory.c        |  9 +------
>> mm/truncate.c           |  6 +++--
>> 3 files changed, 28 insertions(+), 42 deletions(-)
>>
>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>> index c4a811958cda..3d9587f40c0b 100644
>> --- a/include/linux/huge_mm.h
>> +++ b/include/linux/huge_mm.h
>> @@ -383,45 +383,30 @@ static inline int split_huge_page_to_list_to_order(struct page *page, struct lis
>> }
>>
>> /*
>> - * try_folio_split - try to split a @folio at @page using non uniform split.
>> + * try_folio_split_to_order - try to split a @folio at @page to @new_order using
>> + * non uniform split.
>>  * @folio: folio to be split
>> - * @page: split to order-0 at the given page
>> - * @list: store the after-split folios
>> + * @page: split to @order at the given page
>
> split to @new_order?

Will fix it.

>
>> + * @new_order: the target split order
>>  *
>> - * Try to split a @folio at @page using non uniform split to order-0, if
>> - * non uniform split is not supported, fall back to uniform split.
>> + * Try to split a @folio at @page using non uniform split to @new_order, if
>> + * non uniform split is not supported, fall back to uniform split. After-split
>> + * folios are put back to LRU list. Use min_order_for_split() to get the lower
>> + * bound of @new_order.
>
> We removed min_order_for_split() here right?

We removed it from the code, but caller should use min_order_for_split()
to get the lower bound of new_order if they do not want to split to fail
unexpectedly.

Thank you for the review.

>
>>  *
>>  * Return: 0: split is successful, otherwise split failed.
>>  */
>> -static inline int try_folio_split(struct folio *folio, struct page *page,
>> -		struct list_head *list)
>> +static inline int try_folio_split_to_order(struct folio *folio,
>> +		struct page *page, unsigned int new_order)
>> {
>> -	int ret = min_order_for_split(folio);
>> -
>> -	if (ret < 0)
>> -		return ret;
>> -
>> -	if (!non_uniform_split_supported(folio, 0, false))
>> -		return split_huge_page_to_list_to_order(&folio->page, list,
>> -				ret);
>> -	return folio_split(folio, ret, page, list);
>> +	if (!non_uniform_split_supported(folio, new_order, /* warns= */ false))
>> +		return split_huge_page_to_list_to_order(&folio->page, NULL,
>> +				new_order);
>> +	return folio_split(folio, new_order, page, NULL);
>> }
>
> -- 
> Wei Yang
> Help you, Help me


--
Best Regards,
Yan, Zi