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

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Message-ID: <CAGsJ_4yw4Qn7OWmUJeJZYuOHLjJ7L8kov3Th3vFPa3h4y0ur_A@mail.gmail.com>
Date: Mon, 4 Aug 2025 21:15:07 +1200
From: Barry Song <21cnbao@...il.com>
To: "Lai, Yi" <yi1.lai@...ux.intel.com>
Cc: David Hildenbrand <david@...hat.com>, Qi Zheng <zhengqi.arch@...edance.com>, 
	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 Mon, Aug 4, 2025 at 8:49 PM Lai, Yi <yi1.lai@...ux.intel.com> wrote:
>
> On Mon, Aug 04, 2025 at 10:30:45AM +0200, David Hildenbrand wrote:
> > 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.
> >
> >
> >
> My kernel compiling steps:
> 1. copy kconfig_origin to kernel_source_folder/.config
> 2. make olddefconfig
> 3. make bzImage -jx
>
> I have also uploaded the actual .config during compiling.
> [2] https://github.com/laifryiee/syzkaller_logs/blob/main/250803_193026___pte_offset_map_lock/.config
> CONFIG_ARCH_SUPPORTS_PT_RECLAIM=y
> CONFIG_PT_RECLAIM=y

Thanks! I can reproduce the issue within one second.
After disabling PT_RECLAIM in .config, the issue disappears.
The reason it doesn't occur on arm64 is that x86 is the only platform
that supports ARCH_SUPPORTS_PT_RECLAIM.

>
> > [1] https://github.com/laifryiee/syzkaller_logs/tree/main/250803_193026___pte_offset_map_lock/kconfig_origin
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >

Thanks
Barry