Re: [PATCH v4] mm: use per_vma lock for MADV_DONTNEED

["Lai, Yi" <yi1.lai@linux.intel.com>]

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