linux-kernel - Re: [PATCH v4] mm: use per_vma lock for MADV

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Message-ID: <d9d0890e-2837-49dc-9ca8-1d6c28e95a21@redhat.com>
Date: Mon, 4 Aug 2025 10:30:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, "Lai, Yi"
 <yi1.lai@...ux.intel.com>, Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Lokesh Gidra
 <lokeshgidra@...gle.com>, Tangquan Zheng <zhengtangquan@...o.com>,
 yi1.lai@...el.com
Subject: Re: [PATCH v4] mm: use per_vma lock for MADV_DONTNEED

On 04.08.25 10:26, Qi Zheng wrote:
> 
> 
> On 8/4/25 3:57 PM, David Hildenbrand wrote:
>> On 04.08.25 02:58, Lai, Yi wrote:
>>> Hi Barry Song,
>>>
>>> Greetings!
>>>
>>> I used Syzkaller and found that there is general protection fault in
>>> __pte_offset_map_lock in linux-next next-20250801.
>>>
>>> After bisection and the first bad commit is:
>>> "
>>> a6fde7add78d mm: use per_vma lock for MADV_DONTNEED
>>> "
>>>
>>> All detailed into can be found at:
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock
>>> Syzkaller repro code:
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock/repro.c
>>> Syzkaller repro syscall steps:
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock/repro.prog
>>> Syzkaller report:
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock/repro.report
>>> Kconfig(make olddefconfig):
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock/kconfig_origin
>>> Bisect info:
>>> https://github.com/laifryiee/syzkaller_logs/tree/
>>> main/250803_193026___pte_offset_map_lock/bisect_info.log
>>> bzImage:
>>> https://github.com/laifryiee/syzkaller_logs/raw/refs/heads/
>>> main/250803_193026___pte_offset_map_lock/bzImage_next-20250801
>>> Issue dmesg:
>>> https://github.com/laifryiee/syzkaller_logs/blob/
>>> main/250803_193026___pte_offset_map_lock/next-20250801_dmesg.log
>>
>> Skimming over the reproducer, we seem to have racing MADV_DONTNEED and
>> MADV_COLLAPSE on the same anon area, but the problem only shows up once
>> we tear down that MM.
>>
>> If I would have to guess, I'd assume it's related to PT_RECLAIM
>> reclaiming empty page tables during MADV_DONTNEED -- but the kconfig
>> does not indicate that CONFIG_PT_RECLAIM was set.
> 
> On the x86_64, if PT_RECLAIM is not manually disabled, PT_RECLAIM should
> be enabled

That's what I thought: but I was not able to spot it in the provided 
config [1].

Or is that config *before* "make olfconfig"? confusing. I would want to 
see the actually used config.



[1] 
https://github.com/laifryiee/syzkaller_logs/tree/main/250803_193026___pte_offset_map_lock/kconfig_origin

-- 
Cheers,

David / dhildenb