linux-kernel - Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio

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Date: Wed, 5 Jun 2024 16:42:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, ran xiaokai <ranxiaokai627@....com>
Cc: 21cnbao@...il.com, akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
 v-songbaohua@...o.com, xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH linux-next] mm: huge_memory: fix misused
 mapping_large_folio_support() for anon folios

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@....com.cn> wrote:
>>>>
>>>> From: Ran Xiaokai <ran.xiaokai@....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@....com.cn>
>>>> Cc: xu xin <xu.xin16@....com.cn>
>>>> Cc: Yang Yang <yang.yang29@....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.

-- 
Cheers,

David / dhildenb