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Message-ID: <064cca31-442d-4847-b353-26dc5fd0603c@bytedance.com>
Date: Tue, 5 Aug 2025 17:30:52 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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 8/5/25 4:56 PM, Baolin Wang wrote:
>
>
> 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 is no longer valid.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.
CPU0 CPU1
==== ====
collapse
--> check_pmd_still_valid
vma read lock
pt_reclaim clear the pmd entry and will
free the PTE page (via RCU)
vma read unlock
vma write lock
_pmd = pmdp_collapse_flush(vma, address, pmd) <-- pmd_none(_pmd)
pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); <-- pte is
NULL
pmd_populate(mm, pmd, pmd_pgtable(_pmd)); <-- populate a valid page?
vma write unlock
The above is the concurrent scenario you mentioned, right?
What types of this 'valid page' could be? If __pte_offset_map() returns
non-NULL, then it is a PTE page. Even if it is not the original one, it
should not cause panic. Did I miss some key information? :(
>
>> 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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