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

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Message-ID: <FB244ACA-0356-41AB-9735-D98B0452CEE9@nvidia.com>
Date: Mon, 13 Oct 2025 13:11:35 -0400
From: Zi Yan <ziy@...dia.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: linmiaohe@...wei.com, david@...hat.com, jane.chu@...cle.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
Subject: Re: [PATCH 1/2] mm/huge_memory: do not change split_huge_page*()
 target order silently.

On 12 Oct 2025, at 4:24, Pankaj Raghav (Samsung) wrote:

> On Fri, Oct 10, 2025 at 01:39:05PM -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.
>>
>> 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>
>> ---
> LGTM with the suggested changes to the !CONFIG_THP try_folio_split().
>
> Reviewed-by: Pankaj Raghav <p.raghav@...sung.com>

Thanks.

--
Best Regards,
Yan, Zi