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

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Message-ID: <90cd5be7-cbe6-42c5-b0eb-bf9613047097@pankajraghav.com>
Date: Tue, 2 Sep 2025 14:15:42 +0200
From: Pankaj Raghav <kernel@...kajraghav.com>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: David Hildenbrand <david@...hat.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>, 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 9/2/25 13:22, Kiryl Shutsemau wrote:
> On Tue, Sep 02, 2025 at 10:40:36AM +0200, 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.
> 
> Other view is that the page is already split therefore nop.
> 
>> 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.
> 
> Could you elaborate on the undefined behaviour? I don't see it.
> 
> If you argue that this code should not be reachable on !THP config, add
> WARN() there. But I don't see a value.

Little bit of context:

I started investigating what it takes to remove large folio dependency on THP[1][2]

I have some non-upstream changes which enables Large block size (therefore it uses large folios) on
systems with !CONFIG_THP.

I was hitting a weird stale content read error and finally ended up with this fix.

I thought this is a self-contained patch that can already be upstream. My argument is not that this
should not be reachable, but returning -EINVAL will do the right thing instead of returning 0, which
means success.

I hope it clarifies a bit. Let me know what you think.

[1] https://lore.kernel.org/linux-mm/731d8b44-1a45-40bc-a274-8f39a7ae0f7f@lucifer.local/
[2] https://lore.kernel.org/all/aGfNKGBz9lhuK1AF@casper.infradead.org/
--
Pankaj