linux-kernel - Re: [PATCH v2] huge_mm.h: disallow is_huge_zero

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Message-ID: <4927ed52-a250-4ae8-b596-6f81020eb31a@redhat.com>
Date: Wed, 27 Aug 2025 22:26:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Max Kellermann <max.kellermann@...os.com>
Cc: akpm@...ux-foundation.org, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, shikemeng@...weicloud.com,
 kasong@...cent.com, nphamcs@...il.com, bhe@...hat.com, chrisl@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] huge_mm.h: disallow is_huge_zero_folio(NULL)

On 27.08.25 22:01, David Hildenbrand wrote:
> 
>> This seems not so great.
>>
>> I really think the VM_WARN_ON_ONCE() should be in folios_put_refs() based
>> on what you've said.
> 
> No.
> 
> Everything in folio_put_refs() will dereference the folio and properly
> crash the machine when doing something stupid.
> 
> This function, however, will silently swallow a "NULL" pointer.
> 
>>
>>>
>>> Signed-off-by: Max Kellermann <max.kellermann@...os.com>
>>> ---
>>>    include/linux/huge_mm.h | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index 7748489fde1b..bc9ca7798f2e 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -2,6 +2,7 @@
>>>    #ifndef _LINUX_HUGE_MM_H
>>>    #define _LINUX_HUGE_MM_H
>>>
>>> +#include <linux/mmdebug.h> // for VM_WARN_ON_ONCE()
>>
>> Please don't do //.
>>
>> This include is suspect though, huge_mm.h is included from mm.h and thus
>> this very easily might break some arch that is weird about this stuff,
>> because a ton of stuff includes mm.h including things that might absolutely
>> baulk at mmdebug.
> 
> Jup. Very likely this is not required.

Took another look and including mmdebug.h should likely be fine, because 
all it includes is really

(1) include/linux/bug.h
(2) include/linux/stringify.h

But originally, I wonder if we even need mmdebug.h or if it is already 
implicitly included.

huge_mm.h includes mm_types.h (where we already do use VM_WARN...).

I assume there we get it implicitly through percpu.h, mmap_lock.h or 
percpu.h

So the include should just get dropped.

-- 
Cheers

David / dhildenb