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

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Message-ID: <e64diq7jjemybcwr2kgmfrp7xxj6osfdnjmpozilhyjjrt4g6m@brocsk7dnbgp>
Date: Sun, 12 Oct 2025 10:24:19 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.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 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>