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Message-ID: <c6f9dacf-d520-4cc3-88ac-c5937916aa2c@linux.alibaba.com>
Date: Tue, 5 Aug 2025 16:56:22 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, Barry Song <21cnbao@...il.com>,
akpm@...ux-foundation.org, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
"Lai, Yi" <yi1.lai@...ux.intel.com>, David Hildenbrand <david@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...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>,
Lance Yang <ioworker0@...il.com>, Zi Yan <ziy@...dia.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
<npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
Dev Jain <dev.jain@....com>
Subject: Re: [PATCH] mm: Fix the race between collapse and PT_RECLAIM under
per-vma lock
On 2025/8/5 16:17, Qi Zheng wrote:
> Hi Baolin,
>
> On 8/5/25 3:53 PM, Baolin Wang wrote:
>>
>>
>> On 2025/8/5 14:42, Qi Zheng wrote:
>>> Hi Barry,
>>>
>>> On 8/5/25 11:54 AM, Barry Song wrote:
>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>
>>>> The check_pmd_still_valid() call during collapse is currently only
>>>> protected by the mmap_lock in write mode, which was sufficient when
>>>> pt_reclaim always ran under mmap_lock in read mode. However, since
>>>> madvise_dontneed can now execute under a per-VMA lock, this assumption
>>>> is no longer valid. As a result, a race condition can occur between
>>>> collapse and PT_RECLAIM, potentially leading to a kernel panic.
>>>
>>> There is indeed a race condition here. And after applying this patch, I
>>> can no longer reproduce the problem locally (I was able to reproduce it
>>> stably locally last night).
>>>
>>> But I still can't figure out how this race condtion causes the
>>> following panic:
>>>
>>> exit_mmap
>>> --> mmap_read_lock()
>>> unmap_vmas()
>>> --> pte_offset_map_lock
>>> --> rcu_read_lock()
>>> check if the pmd entry is a PTE page
>>> ptl = pte_lockptr(mm, &pmdval) <-- ptl is NULL
>>> spin_lock(ptl) <-- PANIC!!
>>>
>>> If this PTE page is freed by pt_reclaim (via RCU), then the ptl can
>>> not be NULL.
>>>
>>> The collapse holds mmap write lock, so it is impossible to be concurrent
>>> with exit_mmap().
>>>
>>> Confusing. :(
>>
>> IIUC, the issue is not caused by the concurrency between exit_mmap and
>> collapse, but rather by the concurrency between pt_reclaim and collapse.
>>
>> Before this patch, khugepaged might incorrectly restore a PTE
>> pagetable that had already been freed.
>>
>> pt_reclaim has cleared the pmd entry and freed the PTE page table.
>> However, due to the race condition, check_pmd_still_valid() still
>> passes and continues to attempt the collapse:
>>
>> _pmd = pmdp_collapse_flush(vma, address, pmd); ---> returns a none pmd
>> entry (the original pmd entry has been cleared)
>>
>> pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); ---> returns
>> pte == NULL
>>
>> Then khugepaged will restore the old PTE pagetable with an invalid pmd
>> entry:
>>
>> pmd_populate(mm, pmd, pmd_pgtable(_pmd));
>>
>> So when the process exits and trys to free the mapping of the process,
>> traversing the invalid pmd table will lead to a crash.
>
> CPU0 CPU1
> ==== ====
>
> collapse
> --> pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> mmap_write_unlock
> exit_mmap
> --> hold mmap lock
> __pte_offset_map_lock
> --> pte = __pte_offset_map(pmd, addr,
> &pmdval);
> if (unlikely(!pte))
> return pte; <-- will return
__pte_offset_map() might not return NULL? Because the 'pmd_populate(mm,
pmd, pmd_pgtable(_pmd))' could populate a valid page (although the
'_pmd' entry is NONE), but it is not the original pagetable page.
> IIUC, in this case, if we get an invalid pmd entry, we will retrun
> directly instead of causing a crash?
>
>>
>> Barry, please correct me if I have misunderstood something.
>>
>
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