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

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Message-ID: <40f4a688-42e3-4556-979a-209f0ab24c2a@redhat.com>
Date: Tue, 2 Sep 2025 11:06:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Pankaj Raghav <kernel@...kajraghav.com>,
 Ryan Roberts <ryan.roberts@....com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 Nico Pache <npache@...hat.com>, Zi Yan <ziy@...dia.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, willy@...radead.org,
 mcgrof@...nel.org, gost.dev@...sung.com, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH] huge_memory: return -EINVAL in folio split functions when
 THP is disabled

On 02.09.25 10:59, Pankaj Raghav wrote:
> 
> On 9/2/25 10:43, David Hildenbrand wrote:
>> On 02.09.25 10:40, 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.
>>>
>>> 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.
>>
>> Currently large folios that could be split are impossible without THP, so why should this be a fix?
>>
> I was debating with myself whether it will classify as a fix or not. I have had people tell me that
> I should mention it as fix even though I didn't feel like it ;)
> 
> But I agree with you, we can't hit this issue in the upstream kernel. I can remove the fixes tag in
> the next version.

No need to resend, I think Andrew can just drop the comment.

I'll not that I assume with any large folio support (except unsplittable 
things like hugetlb), we would require splitting support.

So calling these functions would rather indicate a bug I think.

Fine with making them return -EINVAL.

With the Fixes: tag dropped

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb