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

[Jinjiang Tu <tujinjiang@huawei.com>]

在 2026/2/25 17:15, David Hildenbrand (Arm) 写道:
> On 2/25/26 09:12, Jinjiang Tu 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 general, relying on a "struct page" for a migration entry is shaky,
> because it can change any time from being a large folio to being a small
> folio.
>
> So we generally only check properties that would be true for either the
> old (large) or the new (smaller) folio, like folio_test_ksm() or
> folio_test_anon().
>
> It's important that these properties were written for the new folio
> before a migration entry user might look at the page indeed.
>
> So it's not just about the locked state.
>
>> 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.
>>
>> Although the code exists for long time, this issue should only exist after
>> mTHP splitting is supported. For THP splitting, there is only a pmd
>> migration entry
> When splitting, we first install a PTE table, no?
>
> unmap_folio() passes TTU_SPLIT_HUGE_PMD.
>
> and it's impossible to access migration entry that stores

Indeed, I misunderstood the code. So the fix tag is incorrect.

>> tail page pfn.
>>
>> To fix it, add missing smp_rmb() if the softleaf entry is migration entry
>> in softleaf_to_folio() and softleaf_to_page().
>>
>> Fixes: 7dc7c5ef6463 ("mm: allow deferred splitting of arbitrary anon large folios")
>> Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
>> ---
>>   include/linux/leafops.h | 39 ++++++++++++++++++++++++++-------------
>>   1 file changed, 26 insertions(+), 13 deletions(-)
>>
>> diff --git a/include/linux/leafops.h b/include/linux/leafops.h
>> index a9ff94b744f2..f823f390ba6b 100644
>> --- a/include/linux/leafops.h
>> +++ b/include/linux/leafops.h
>> @@ -371,14 +371,21 @@ static inline unsigned long softleaf_to_pfn(softleaf_t entry)
>>    */
>>   static inline struct page *softleaf_to_page(softleaf_t entry)
>>   {
>> -	struct page *page = pfn_to_page(softleaf_to_pfn(entry));
>> +	struct page *page;
>>   
>>   	VM_WARN_ON_ONCE(!softleaf_has_pfn(entry));
>> -	/*
>> -	 * Any use of migration entries may only occur while the
>> -	 * corresponding page is locked
>> -	 */
>> -	VM_WARN_ON_ONCE(softleaf_is_migration(entry) && !PageLocked(page));
>> +
>> +	page = pfn_to_page(softleaf_to_pfn(entry));
>> +	if (softleaf_is_migration(entry)) {
>> +		/* See __split_folio_to_order() comment */
>> +		smp_rmb();
>> +
>> +		/*
>> +		 * Any use of migration entries may only occur while the
>> +		 * corresponding page is locked
>> +		 */
>> +		VM_WARN_ON_ONCE(!PageLocked(page));
>> +	}
> Conceptually, wouldn't the smb_rmb() have to happen *after* the
> page_folio(), like we have in softleaf_to_folio()?

the comments of page_folio() says:
  * Context: No reference, nor lock is required on @page.  If the caller
  * does not hold a reference, this call may race with a folio split, so
  * it should re-check the folio still contains this page after gaining
  * a reference on the folio.
  * Return: The folio which contains this page.

The old large folio is locked and freezed during splitting, page_folio()
couldn't be called with reference held, so the result is unstable, the
caller should recheck after gaining a reference, at that time splitting
finishes, and folio_get() contains memory barrier already, ensuring the
folio flags is seen after compound_head.

>