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

[Zi Yan]

On 6 Jun 2024, at 14:00, 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.

>
> 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

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