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

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Message-ID: <CAGsJ_4ypaAySaWyatH4Vu_dz--gEXUWk3yAQwM_8JTdR6U3fQA@mail.gmail.com>
Date: Tue, 5 Aug 2025 09:48:32 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>, "Lai, Yi" <yi1.lai@...ux.intel.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 Mon, Aug 4, 2025 at 7:57 PM David Hildenbrand <david@...hat.com> 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:
[...]
>
> 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.
>

This seems to be where the race happens.
Hi Lai, can you also double check if the below diff fixes the problem?

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 374a6a5193a7..6b40bdfd224c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1172,11 +1172,11 @@ static int collapse_huge_page(struct mm_struct
*mm, unsigned long address,
        if (result != SCAN_SUCCEED)
                goto out_up_write;
        /* check if the pmd is still valid */
+       vma_start_write(vma);
        result = check_pmd_still_valid(mm, address, pmd);
        if (result != SCAN_SUCCEED)
                goto out_up_write;

-       vma_start_write(vma);
        anon_vma_lock_write(vma->anon_vma);

        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, mm, address,

> 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
>

Thanks
Barry