linux-kernel - Re: [PATCH v2 3/5] mm/huge_memory.c: add missing read-only THP checking in transparent_hugepage

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Date:   Fri, 30 Apr 2021 09:49:08 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     ziy@...dia.com, william.kucharski@...cle.com, willy@...radead.org,
        yang.shi@...ux.alibaba.com, aneesh.kumar@...ux.ibm.com,
        rcampbell@...dia.com, songliubraving@...com,
        kirill.shutemov@...ux.intel.com, riel@...riel.com,
        hannes@...xchg.org, minchan@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 3/5] mm/huge_memory.c: add missing read-only THP
 checking in transparent_hugepage_enabled()

On 30.04.21 03:57, Miaohe Lin wrote:
> On 2021/4/29 22:57, David Hildenbrand wrote:
>> On 29.04.21 15:26, Miaohe Lin 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().
>>> To fix it, we add checking for read-only THP file mapping and also
>>> introduce helper transhuge_vma_enabled() to check whether thp is
>>> enabled for specified vma to reduce duplicated code.
>>>
>>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>    include/linux/huge_mm.h | 21 +++++++++++++++++----
>>>    mm/huge_memory.c        |  6 ++++++
>>>    mm/khugepaged.c         |  4 +---
>>>    mm/shmem.c              |  3 +--
>>>    4 files changed, 25 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index 0a526f211fec..f460b74619fc 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -115,6 +115,16 @@ extern struct kobj_attribute shmem_enabled_attr;
>>>      extern unsigned long transparent_hugepage_flags;
>>>    +static inline bool transhuge_vma_enabled(struct vm_area_struct *vma,
>>> +                      unsigned long vm_flags)
>>
>> You're passing the vma already, why do you pass vma->vm_flags separately? It's sufficient to pass in the vma only.
>>
> 
> Many thanks for comment! IMO, vm_flags may not always equal to vma->vm_flags. When hugepage_vma_check()
> is called from collapse_pte_mapped_thp, vma_flags = vma->vm_flags | VM_HUGEPAGE. So I think we should
> pass vm_flags here.

Oh, sorry, I missed the hugepage_vma_check() user. That's unfortunate.

>>>    static inline void prep_transhuge_page(struct page *page) {}
>>>      static inline bool is_transparent_hugepage(struct page *page)
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 76ca1eb2a223..e24a96de2e37 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -68,12 +68,18 @@ bool transparent_hugepage_enabled(struct vm_area_struct *vma)
>>>        /* The addr is used to check if the vma size fits */
>>>        unsigned long addr = (vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE;
>>>    +    if (!transhuge_vma_enabled(vma, vma->vm_flags))
>>> +        return false;
>>>        if (!transhuge_vma_suitable(vma, addr))
>>>            return false;
>>>        if (vma_is_anonymous(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;
>>
>> Nit: I'm really wondering why we have 3 different functions that sound like they are doing the same thing
>>
>> transparent_hugepage_enabled(vma)
>> transhuge_vma_enabled()
>> transhuge_vma_suitable()
>>
>> Which check belongs where? Does it really have to be that complicated?
>>
> 
> IMO, transhuge_vma_suitable() checks whether pgoff , vm_start and vm_end is possible for thp.
> transhuge_vma_enabled() checks whether thp is explicitly disabled through madvise.
> And transparent_hugepage_enabled() use these helpers to get the conclusion whether thp is
> enabled for specified vma.
> 
> Any suggestions?

transparent_hugepage_enabled() vs. transhuge_vma_enabled() is really 
sub-optimal naming. I guess "transparent_hugepage_active()" would have 
been clearer (enabled + suitable + applicable). Cannot really give a 
good suggestion here on how to name transhuge_vma_enabled() differently.


We now have

transparent_hugepage_enabled()
-> transhuge_vma_enabled()
-> __transparent_hugepage_enabled() -> transhuge_vma_enabled()
-> shmem_huge_enabled() -> transhuge_vma_enabled()

That looks sub-optimal as well. Maybe we should have a

static inline bool file_thp_enabled(struct vma *vma)
{
	return transhuge_vma_enabled() &&
	       IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
	       !inode_is_open_for_write(vma->vm_file->f_inode) &&
	       (vma->vm_flags & VM_EXEC))
}

and in transparent_hugepage_enabled() only do a

if (vma->vm_file)
	return file_thp_enabled(vma);


Or move the transhuge_vma_enabled() check completely to 
transparent_hugepage_enabled() if possible.

-- 
Thanks,

David / dhildenb