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

[Pankaj Raghav <kernel@pankajraghav.com>]

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@samsung.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