Re: [PATCH 3/5] mm/huge_memory.c: add missing read-only THP checking in transparent_hugepage_enabled()

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/28 5:03, Yang Shi wrote:
> On Tue, Apr 27, 2021 at 6:32 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>
>> Since commit 99cb0dbd47a1 ("mm,thp: add read-only THP support for
>> (non-shmem) FS"), read-only THP file mapping is supported. But it
>> forgot to add checking for it in transparent_hugepage_enabled().
>>
>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> ---
>>  mm/huge_memory.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 76ca1eb2a223..aa22a0ae9894 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -74,6 +74,9 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>                 return __transparent_hugepage_enabled(vma);
>>         if (vma_is_shmem(vma))
>>                 return shmem_huge_enabled(vma);
>> +       if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
>> +           (vma->vm_flags & VM_DENYWRITE))
>> +               return true;
> 

Many thanks for your quick respond and Reviewed-by tag!

> I don't think this change is correct. This function is used to
> indicate if allocating THP is eligible for the VMAs or not showed by
> smap. And currently readonly FS THP is collapsed by khugepaged only.
> 
> So, you need check if the vma is suitable for khugepaged. Take a look
> at what hugepage_vma_check() does.
> 
> And, the new patch
> (https://lore.kernel.org/linux-mm/20210406000930.3455850-1-cfijalkovich@google.com/)
> relax the constraints for readonly FS THP, it might be already in -mm
> tree, so you need adopt the new condition as well.
> 

Many thanks for your comment. I referred to what hugepage_vma_check() does about
Read-only file mappings when I came up this patch. But it seems I am miss something.
Take the new patch into account, the check for READ_ONLY_THP now should be:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 76ca1eb2a223..a46a558233b4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -74,6 +74,10 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
                return __transparent_hugepage_enabled(vma);
        if (vma_is_shmem(vma))
                return shmem_huge_enabled(vma);
+       if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file &&
+           !inode_is_open_for_write(vma->vm_file->f_inode) &&
+           (vma->vm_flags & VM_EXEC))
+               return true;

        return false;
 }

Am I miss something about checking for READ_ONLY_THP case? Or READ_ONLY_THP case is ok
but other case is missed? Could you please explain this more detailed for me?

Many thanks!

>>
>>         return false;
>>  }
> 
>> --
>> 2.23.0
>>
>>
> 
> .
>