linux-kernel - Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

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Date:   Tue, 16 Oct 2018 18:46:48 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     "Aneesh Kumar K.V" <aneeshkumar.opensource@...il.com>,
        Will Deacon <will.deacon@....com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kirill.shutemov@...ux.intel.com, akpm@...ux-foundation.org,
        mhocko@...e.com, zi.yan@...rutgers.edu
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and
 PMD migration entry



On 10/15/2018 02:02 PM, Aneesh Kumar K.V wrote:
> On 10/12/18 1:32 PM, Anshuman Khandual wrote:
>>
>>
>> On 10/09/2018 06:48 PM, Will Deacon wrote:
>>> On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote:
>>>> On Tue, Oct 09, 2018 at 09:28:58AM +0530, Anshuman Khandual wrote:
>>>>> A normal mapped THP page at PMD level should be correctly differentiated
>>>>> from a PMD migration entry while walking the page table. A mapped THP would
>>>>> additionally check positive for pmd_present() along with pmd_trans_huge()
>>>>> as compared to a PMD migration entry. This just adds a new conditional test
>>>>> differentiating the two while walking the page table.
>>>>>
>>>>> Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path")
>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>>>> ---
>>>>> On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually
>>>>> exclusive which makes the current conditional block work for both mapped
>>>>> and migration entries. This is not same with arm64 where pmd_trans_huge()
>>>>> returns positive for both mapped and migration entries. Could some one
>>>>> please explain why pmd_trans_huge() has to return false for migration
>>>>> entries which just install swap bits and its still a PMD ?
>>>>
>>>> I guess it's just a design choice. Any reason why arm64 cannot do the
>>>> same?
>>>
>>> Anshuman, would it work to:
>>>
>>> #define pmd_trans_huge(pmd)     (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
>> yeah this works but some how does not seem like the right thing to do
>> but can be the very last option.
>>
> 
> 
> There can be other code paths that makes that assumption. I ended up doing the below for pmd_trans_huge on ppc64.
>

Yeah, did see that in one of the previous proposals.

https://patchwork.kernel.org/patch/10544291/

But the existing semantics does not look right and makes vague assumptions.
Zi Yan has already asked Andrea for his input in this regard on the next
thread. So I guess while being here, its a good idea to revisit existing
semantics and it's assumptions before fixing it in arch specific helpers.

- Anshuman 


> /*
>  * Only returns true for a THP. False for pmd migration entry.
>  * We also need to return true when we come across a pte that
>  * in between a thp split. While splitting THP, we mark the pmd
>  * invalid (pmdp_invalidate()) before we set it with pte page
>  * address. A pmd_trans_huge() check against a pmd entry during that time
>  * should return true.
>  * We should not call this on a hugetlb entry. We should check for HugeTLB
>  * entry using vma->vm_flags
>  * The page table walk rule is explained in Documentation/vm/transhuge.rst
>  */
> static inline int pmd_trans_huge(pmd_t pmd)
> {
>     if (!pmd_present(pmd))
>         return false;
> 
>     if (radix_enabled())
>         return radix__pmd_trans_huge(pmd);
>     return hash__pmd_trans_huge(pmd);
> }
> 
> -aneesh
>