Re: [PATCH v3] mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/20/26 02:52, Jinjiang Tu wrote:
> 
> 在 2026/3/20 6:51, Andrew Morton 写道:
>> On Thu, 19 Mar 2026 09:25:41 +0800 Jinjiang Tu <tujinjiang@huawei.com>
>> wrote:
>>
>>> On arm64 server, we found folio that get from migration entry isn't
>>> locked
>>> in softleaf_to_folio(). This issue triggers when mTHP splitting and
>>> zap_nonpresent_ptes() races, and the root cause is lack of memory
>>> barrier
>>> in softleaf_to_folio(). The race is as follows:
>>>
>>>     CPU0                                             CPU1
>>>
>>> deferred_split_scan()                              zap_nonpresent_ptes()
>>>    lock folio
>>>    split_folio()
>>>      unmap_folio()
>>>        change ptes to migration entries
>>>      __split_folio_to_order()                        
>>> softleaf_to_folio()
>>>        set flags(including PG_locked) for tail pages    folio =
>>> pfn_folio(softleaf_to_pfn(entry))
>>>        smp_wmb()                                       
>>> VM_WARN_ON_ONCE(!folio_test_locked(folio))
>>>        prep_compound_page() for tail pages
>>>
>>> In __split_folio_to_order(), smp_wmb() guarantees page flags of tail
>>> pages
>>> are visible before the tail page becomes non-compound. smp_wmb() should
>>> be paired with smp_rmb() in softleaf_to_folio(), which is missed. As a
>>> result, if zap_nonpresent_ptes() accesses migration entry that stores
>>> tail pfn, softleaf_to_folio() may see the updated compound_head of tail
>>> page before page->flags.
>> Please describe the userspace-visible runtime effects of this bug.
> 
> This issue will trigger VM_WARN_ON_ONCE() in pfn_swap_entry_folio().

But the impact is bigger, right, when callers rely on folio_test_anon() etc?

-- 
Cheers,

David