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

[Zi Yan]

On 5 Jun 2024, at 2:54, ran xiaokai wrote:

>> On Tue, Jun 4, 2024 at 5:47?PM <xu.xin16@xxxxxxxxxx> wrote:
>>>
>>> From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> Cc: xu xin <xu.xin16@xxxxxxxxxx>
>>> Cc: Yang Yang <yang.yang29@xxxxxxxxxx>
>>> ---
>>>  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?

Best Regards,
Yan, Zi

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