Re: [PATCH linux-next v2] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

[Zi Yan]

On 6 Jun 2024, at 14:33, Barry Song wrote:

> On Fri, Jun 7, 2024 at 9:24 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>>
>> On 6 Jun 2024, at 14:00, Barry Song wrote:
>>
>>> On Fri, Jun 7, 2024 at 2:35 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
>>>>
>>>> +Matthew
>>>>
>>>> For mapping_large_folio_support() changes.
>>>>
>>>> On 6 Jun 2024, at 2:42, 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 the 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.
>>>>>
>>>>> Also add a VM_WARN_ON_ONCE() in mapping_large_folio_support()
>>>>> for anon mapping, So we can detect the wrong use more easily.
>>>>>
>>>>> THP folios maybe exist in the pagecache even the file system doesn't
>>>>> support large folio, it is because when CONFIG_TRANSPARENT_HUGEPAGE
>>>>> is enabled, khugepaged will try to collapse read-only file-backed pages
>>>>> to THP. But the mapping does not actually support multi order
>>>>> large folios properly.
>>>>>
>>>>> 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>
>>>>> ---
>>>>>  include/linux/pagemap.h |  4 ++++
>>>>>  mm/huge_memory.c        | 27 ++++++++++++++++-----------
>>>>>  2 files changed, 20 insertions(+), 11 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>>>> index ee633712bba0..59f1df0cde5a 100644
>>>>> --- a/include/linux/pagemap.h
>>>>> +++ b/include/linux/pagemap.h
>>>>> @@ -381,6 +381,10 @@ static inline void mapping_set_large_folios(struct address_space *mapping)
>>>>>   */
>>>>>  static inline bool mapping_large_folio_support(struct address_space *mapping)
>>>>>  {
>>>>> +     /* AS_LARGE_FOLIO_SUPPORT is only reasonable for pagecache folios */
>>>>> +     VM_WARN_ONCE((unsigned long)mapping & PAGE_MAPPING_ANON,
>>>>> +                     "Anonymous mapping always supports large folio");
>>>>> +
>>>>>       return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
>>>>>               test_bit(AS_LARGE_FOLIO_SUPPORT, &mapping->flags);
>>>>>  }
>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>> index 317de2afd371..62d57270b08e 100644
>>>>> --- a/mm/huge_memory.c
>>>>> +++ b/mm/huge_memory.c
>>>>> @@ -3009,30 +3009,35 @@ 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))
>>>>> +     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 if (new_order) {
>>>>>               /* 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)) {
>>>>> +             /* No split if the file system does not support large folio.
>>>>> +              * 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.
>>>>> +              */
>>>>> +             if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>>>>> +                     !mapping_large_folio_support(folio->mapping)) {
>>>>
>>>> Shouldn’t this be
>>>>
>>>> if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>>>>         !mapping_large_folio_support(folio->mapping)) {
>>>>
>>>> ?
>>>>
>>>> When CONFIG_READ_ONLY_THP_FOR_FS is not set, we need to check
>>>> mapping_large_folio_support(), otherwise we do not.
>>>
>>> while CONFIG_READ_ONLY_THP_FOR_FS is not set, that is no way
>>> a large folio can be mapped to a filesystem which doesn't support
>>> large folio mapping. i think
>>
>> That is why we have the warning below to catch this undesired
>> case.
>>
>>> if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) is correct.
>>
>> When it is set, khugepaged can create a large pagecache folio
>> on a filesystem without large folio support and the warning
>> will be triggered once the created large pagecache folio
>> is split. That is not what we want.
>
> yes. This is exactly why we need if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS))
> but not if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)) .
>
> because if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS)), folio is definitely
> pointing to a file system supporting large folio. otherwise, it is a bug.

Oh, got it. Thanks for the explanation. :)

>>
>>>
>>> The below means a BUG which has never a chance to happen if it
>>> is true.
>>>
>>> !IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
>>>         !mapping_large_folio_support(folio->mapping));
>>>
>>>>
>>>>>                       VM_WARN_ONCE(1,
>>>>>                               "Cannot split file folio to non-0 order");
>>>>>                       return -EINVAL;
>>>>>               }
>>>>>       }
>>>>>
>>>>> +     /* Only swapping a whole PMD-mapped folio is supported */
>>>>> +     if (folio_test_swapcache(folio) && new_order)
>>>>> +             return -EINVAL;
>>>>>
>>>>>       is_hzp = is_huge_zero_folio(folio);
>>>>>       if (is_hzp) {
>>>>> --
>>>>> 2.15.2
>>>>
>>>>
>>>> Best Regards,
>>>> Yan, Zi
>>>
>>> Thanks
>>> Barry
>>
>>
>> Best Regards,
>> Yan, Zi


Best Regards,
Yan, Zi

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