linux-kernel - Re: [PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in __folio

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Message-ID: <3ecb7e94-c0c2-4dff-98bb-a9e32a48c369@redhat.com>
Date: Fri, 18 Jul 2025 09:22:07 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Dan Carpenter <dan.carpenter@...aro.org>,
 Antonio Quartulli <antonio@...delbit.com>, Hugh Dickins <hughd@...gle.com>,
 Kirill Shutemov <k.shutemov@...il.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>, Balbir Singh <balbirs@...dia.com>,
 Matthew Brost <matthew.brost@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/6] mm/huge_memory: convert VM_BUG* to VM_WARN* in
 __folio_split.

On 18.07.25 04:29, Zi Yan wrote:
> These VM_BUG* can be handled gracefully without crashing kernel.
> 
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>   mm/huge_memory.c | 16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d36f7bdaeb38..d6ff5e8c89d7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3601,8 +3601,14 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>   	pgoff_t end;
>   	bool is_hzp;
>   
> -	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> +	if (!folio_test_locked(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}
> +	if (!folio_test_large(folio)) {
> +		VM_WARN_ON_ONCE_FOLIO(1, folio);
> +		return -EINVAL;
> +	}

For cases that we handle gracefully you usually want to use

if (WARN_ON_ONCE(..))

because then you get actually notified when that unexpected thing happens.

I am not really sure if recovery is warranted here: smells like a 
straight VM_WARN_ON_ONCE_FOLIO() is sufficient, and catching this early 
during development that something is extremely off.

-- 
Cheers,

David / dhildenb