Re: [PATCH v3] mm/shmem: fix THP allocation and fallback loop

[David Hildenbrand <david@redhat.com>]

On 23.10.25 18:13, David Hildenbrand wrote:
> On 23.10.25 08:59, Kairui Song wrote:
>> From: Kairui Song <kasong@tencent.com>
>>
>> The order check and fallback loop is updating the index value on every
>> loop, this will cause the index to be wrongly aligned by a larger value
>> while the loop shrinks the order.
>>
>> This may result in inserting and returning a folio of the wrong index
>> and cause data corruption with some userspace workloads [1].
>>
>> Cc: stable@vger.kernel.org
>> Link: https://lore.kernel.org/linux-mm/CAMgjq7DqgAmj25nDUwwu1U2cSGSn8n4-Hqpgottedy0S6YYeUw@mail.gmail.com/ [1]
>> Fixes: e7a2ab7b3bb5d ("mm: shmem: add mTHP support for anonymous shmem")
>> Signed-off-by: Kairui Song <kasong@tencent.com>
>>
>> ---
>>
>> Changes from V2:
>> - Introduce a temporary variable to improve code,
>>     no behavior change, generated code is identical.
>> - Link to V2: https://lore.kernel.org/linux-mm/20251022105719.18321-1-ryncsn@gmail.com/
>>
>> Changes from V1:
>> - Remove unnecessary cleanup and simplify the commit message.
>> - Link to V1: https://lore.kernel.org/linux-mm/20251021190436.81682-1-ryncsn@gmail.com/
>>
>> ---
>>    mm/shmem.c | 9 ++++++---
>>    1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index b50ce7dbc84a..e1dc2d8e939c 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1882,6 +1882,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
>>    	struct shmem_inode_info *info = SHMEM_I(inode);
>>    	unsigned long suitable_orders = 0;
>>    	struct folio *folio = NULL;
>> +	pgoff_t aligned_index;
>>    	long pages;
>>    	int error, order;
>>    
>> @@ -1895,10 +1896,12 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
>>    		order = highest_order(suitable_orders);
>>    		while (suitable_orders) {
>>    			pages = 1UL << order;
>> -			index = round_down(index, pages);
>> -			folio = shmem_alloc_folio(gfp, order, info, index);
>> -			if (folio)
>> +			aligned_index = round_down(index, pages);
>> +			folio = shmem_alloc_folio(gfp, order, info, aligned_index);
>> +			if (folio) {
>> +				index = aligned_index;
>>    				goto allocated;
>> +			}
> 
> Was the found by code inspection or was there a report about this?

Answering my own question, the "Link:" above should be

Closes: 
https://lore.kernel.org/linux-mm/CAMgjq7DqgAmj25nDUwwu1U2cSGSn8n4-Hqpgottedy0S6YYeUw@mail.gmail.com/


-- 
Cheers

David / dhildenb