Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[David Hildenbrand <david@redhat.com>]

On 06.06.24 03:34, Barry Song wrote:
> On Thu, Jun 6, 2024 at 2:42 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 05.06.24 16:08, Zi Yan wrote:
>>> On 5 Jun 2024, at 2:54, ran xiaokai wrote:
>>>
>>>>> On Tue, Jun 4, 2024 at 5:47?PM <xu.xin16@zte.com.cn> wrote:
>>>>>>
>>>>>> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
>>>>>>
>>>>>> When I did a large folios split test, a WARNING
>>>>>> "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
>>>>>> was triggered. But my test cases are only for anonmous folios.
>>>>>> while mapping_large_folio_support() is only reasonable for page
>>>>>> cache folios.
>>>>>>
>>>>>> In split_huge_page_to_list_to_order(), the folio passed to
>>>>>> mapping_large_folio_support() maybe anonmous folio. The
>>>>>> folio_test_anon() check is missing. So the split of the anonmous THP
>>>>>> is failed. This is also the same for shmem_mapping(). We'd better add
>>>>>> a check for both. But the shmem_mapping() in __split_huge_page() is
>>>>>> not involved, as for anonmous folios, the end parameter is set to -1, so
>>>>>> (head[i].index >= end) is always false. shmem_mapping() is not called.
>>>>>>
>>>>>> Using /sys/kernel/debug/split_huge_pages to verify this, with this
>>>>>> patch, large anon THP is successfully split and the warning is ceased.
>>>>>>
>>>>>> Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
>>>>>> Cc: xu xin <xu.xin16@zte.com.cn>
>>>>>> Cc: Yang Yang <yang.yang29@zte.com.cn>
>>>>>> ---
>>>>>>    mm/huge_memory.c | 38 ++++++++++++++++++++------------------
>>>>>>    1 file changed, 20 insertions(+), 18 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index 317de2afd371..4c9c7e5ea20c 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>>           if (new_order >= folio_order(folio))
>>>>>>                   return -EINVAL;
>>>>>>
>>>>>> -       /* Cannot split anonymous THP to order-1 */
>>>>>> -       if (new_order == 1 && folio_test_anon(folio)) {
>>>>>> -               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
>>>>>> -               return -EINVAL;
>>>>>> -       }
>>>>>> -
>>>>>>           if (new_order) {
>>>>>>                   /* Only swapping a whole PMD-mapped folio is supported */
>>>>>>                   if (folio_test_swapcache(folio))
>>>>>>                           return -EINVAL;
>>>>>> -               /* Split shmem folio to non-zero order not supported */
>>>>>> -               if (shmem_mapping(folio->mapping)) {
>>>>>> -                       VM_WARN_ONCE(1,
>>>>>> -                               "Cannot split shmem folio to non-0 order");
>>>>>> -                       return -EINVAL;
>>>>>> -               }
>>>>>> -               /* No split if the file system does not support large folio */
>>>>>> -               if (!mapping_large_folio_support(folio->mapping)) {
>>>>>> -                       VM_WARN_ONCE(1,
>>>>>> -                               "Cannot split file folio to non-0 order");
>>>>>> -                       return -EINVAL;
>>>>>> +
>>>>>> +               if (folio_test_anon(folio)) {
>>>>>> +                       /* Cannot split anonymous THP to order-1 */
>>>>>> +                       if (new_order == 1) {
>>>>>> +                               VM_WARN_ONCE(1, "Cannot split to order-1 folio");
>>>>>> +                               return -EINVAL;
>>>>>> +                       }
>>>>>> +               } else {
>>>>>> +                       /* Split shmem folio to non-zero order not supported */
>>>>>> +                       if (shmem_mapping(folio->mapping)) {
>>>>>> +                               VM_WARN_ONCE(1,
>>>>>> +                                       "Cannot split shmem folio to non-0 order");
>>>>>> +                               return -EINVAL;
>>>>>> +                       }
>>>>>> +                       /* No split if the file system does not support large folio */
>>>>>> +                       if (!mapping_large_folio_support(folio->mapping)) {
>>>>>> +                               VM_WARN_ONCE(1,
>>>>>> +                                       "Cannot split file folio to non-0 order");
>>>>>> +                               return -EINVAL;
>>>>>> +                       }
>>>>>
>>>>> Am I missing something? if file system doesn't support large folio,
>>>>> how could the large folio start to exist from the first place while its
>>>>> mapping points to a file which doesn't support large folio?
>>>>
>>>> I think it is the CONFIG_READ_ONLY_THP_FOR_FS case.
>>>> khugepaged will try to collapse read-only file-backed pages to 2M THP.
>>>
>>> Can you add this information to the commit log in your next version?
>>
>> Can we also add that as a comment to that function, like "Note that we
>> might still
>> have THPs in such mappings due to CONFIG_READ_ONLY_THP_FOR_FS. But in
>> that case,
>> the mapping does not actually support large folios properly.
>> "Or sth. like that.
> 
> +1. Otherwise, the code appears quite confusing.
> Would using "#ifdef" help to further clarify it?
> 
> #ifdef CONFIG_READ_ONLY_THP_FOR_FS
>              if (!mapping_large_folio_support(folio->mapping)) {
>                            ....
>              }

if (IS_ENABLED()) ...

Agreed.

-- 
Cheers,

David / dhildenb