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

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Message-ID: <1d1d97f9-2a67-4920-850e-accf4c82440e@redhat.com>
Date: Mon, 4 Aug 2025 09:57:34 +0200
From: David Hildenbrand <david@...hat.com>
To: "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>,
 Qi Zheng <zhengqi.arch@...edance.com>, yi1.lai@...el.com
Subject: Re: [PATCH v4] mm: use per_vma lock for MADV_DONTNEED

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.

-- 
Cheers,

David / dhildenb