linux-kernel - Re: [PATCH] mm/huge_memory: Fix iterator variable usage after swap()

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Message-ID: <e47fc0e7-b941-4504-8d69-a937d4d18be5@linux.dev>
Date: Thu, 22 Jan 2026 10:16:36 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 zenghongling <zenghongling@...inos.cn>, Qi Zheng <qi.zheng@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>
Cc: linux-mm@...ck.org, dev.jain@....com, akpm@...ux-foundation.org,
 ryan.roberts@....com, baolin.wang@...ux.alibaba.com, npache@...hat.com,
 linux-kernel@...r.kernel.org, baohua@...nel.org, Liam.Howlett@...cle.com,
 zhongling0719@....com, ziy@...dia.com, lorenzo.stoakes@...cle.com
Subject: Re: [PATCH] mm/huge_memory: Fix iterator variable usage after swap()



On 2026/1/21 20:28, David Hildenbrand (Red Hat) wrote:
> On 1/21/26 10:25, Lance Yang wrote:
>>
>>
>> On 2026/1/21 16:13, zenghongling wrote:
>>> The iterator variable 'folio' is swapped with 'prev' in the else
>>> branch. Using 'folio' after swap() checks the potentially NULL
>>> 'prev' value, not the original iterator value.
>>>
>>> Fix by moving folio_put() call before the swap operation in the
>>> path where swap() occurs.
>>>
>>> Found by:
>>> ./huge_memory.c:4225:6-11: ERROR: iterator variable bound on line 
>>> 4178 cannot be NULL
>>
> 
> Which tool did find that? A compiler?

Looks like a false positive from coccinelle? The pattern is tricky to
follow statically :)

> 
>> Good catch!
>>
>> But which tree is your patch based on?
>>
>> Seems like that was already fixed in commit 776bde7caf80[1]. The
>> whole thing deferred_split_scan() was refactored using folio_batch,
>> so the buggy code with swap(folio, prev) is gone ...
>>
>> Ccing Muchun and Qi who fixed that.
>>
>> [1]
>> https://lore.kernel.org/ 
>> all/59cb6b6fb5ffcff9d23b81890b252960139ad8e7.1762762324.git.zhengqi.arch@...edance.com/
> 
> Right, in
> 
> commit 776bde7caf80f6af72b087cafe7d9f607b14716d
> Author: Muchun Song <muchun.song@...ux.dev>
> Date:   Mon Nov 10 16:17:57 2025 +0800
> 
>      mm: thp: use folio_batch to handle THP splitting in 
> deferred_split_scan()
> 
> 
> Which raises the question whether we would want to backport that patch 
> to stable kernels
> if there was indeed a problem?
> 
> 
> But: I don't immediately see the problem.
> 
> If pref is NULL (and folio obviously !+NULL), we'll end up with
>      * pref != NULL
>      * folio == NULL
> 
>      The "if (folio)" check will do nothing, because we defer the 
> freeing to the
> 
>          if (prev)
>              folio_put(prev);
> 
>      later
> 
> If pref is != NULL (and folio obviously !+NULL), we'll end up with
>      * pref = NULL
>      * folio = NULL
> 
>      The if (folio) and if (prev) handling will care of it all.
> 
> 
> So ... this pretty much looks like working as expected?

Right! After reading through the old code, the swap() design is
correct and working as intended, IIUC.