linux-kernel - Re: [syzbot] [mm?] WARNING in memory

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Message-ID: <w2kwxcd6br6h4tdn6xigtuf73qklt6jhxvhtcwp7idugycgxlv@vqjx26vrnwu5>
Date: Mon, 29 Sep 2025 13:08:47 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, 
	Luis Chamberlain <mcgrof@...nel.org>, syzbot <syzbot+e6367ea2fdab6ed46056@...kaller.appspotmail.com>, 
	akpm@...ux-foundation.org, linmiaohe@...wei.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, nao.horiguchi@...il.com, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] WARNING in memory_failure

> 
> I want to change all the split functions in huge_mm.h and provide
> mapping_min_folio_order() to try_folio_split() in truncate_inode_partial_folio().
> 
> Something like below:
> 
> 1. no split function will change the given order;
> 2. __folio_split() will no longer give VM_WARN_ONCE when provided new_order
> is smaller than mapping_min_folio_order().
> 
> In this way, for an LBS folio that cannot be split to order 0, split
> functions will return -EINVAL to tell caller that the folio cannot
> be split. The caller is supposed to handle the split failure.

IIUC, we will remove warn on once but just return -EINVAL in __folio_split()
function if new_order < min_order like this:
...
		min_order = mapping_min_folio_order(folio->mapping);
		if (new_order < min_order) {
-			VM_WARN_ONCE(1, "Cannot split mapped folio below min-order: %u",
-				     min_order);
			ret = -EINVAL;
			goto out;
		}
...
> 
> WDYT?
> 
I think it should be fine as along as we return an error if someone is
trying to split < min_order for file-backed folios.

> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index f327d62fc985..e15c3ca07e33 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -387,34 +387,16 @@ int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>   * Return: 0: split is successful, otherwise split failed.
>   */
>  static inline int try_folio_split(struct folio *folio, struct page *page,
> -		struct list_head *list)
> +		struct list_head *list, unsigned int order)
>  {
-- 
Pankaj