Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics

[Anshuman Khandual]

On 08/18/2020 02:43 PM, Jonathan Cameron wrote:
> On Mon, 17 Aug 2020 14:49:43 +0530
> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote:
> 
>> pmd_present() and pmd_trans_huge() are expected to behave in the following
>> manner during various phases of a given PMD. It is derived from a previous
>> detailed discussion on this topic [1] and present THP documentation [2].
>>
>> pmd_present(pmd):
>>
>> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
>> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd)
>>
>> pmd_trans_huge(pmd):
>>
>> - Returns true if pmd refers to system RAM and is a trans huge mapping
>>
>> -------------------------------------------------------------------------
>> |	PMD states	|	pmd_present	|	pmd_trans_huge	|
>> -------------------------------------------------------------------------
>> |	Mapped		|	Yes		|	Yes		|
>> -------------------------------------------------------------------------
>> |	Splitting	|	Yes		|	Yes		|
>> -------------------------------------------------------------------------
>> |	Migration/Swap	|	No		|	No		|
>> -------------------------------------------------------------------------
>>
>> The problem:
>>
>> PMD is first invalidated with pmdp_invalidate() before it's splitting. This
>> invalidation clears PMD_SECT_VALID as below.
>>
>> PMD Split -> pmdp_invalidate() -> pmd_mkinvalid -> Clears PMD_SECT_VALID
>>
>> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false
>> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to re-
>> affirm pmd_present() as true during the THP split process. To comply with
>> above mentioned semantics, pmd_trans_huge() should also check pmd_present()
>> first before testing presence of an actual transparent huge mapping.
>>
>> The solution:
>>
>> Ideally PMD_TYPE_SECT should have been used here instead. But it shares the
>> bit position with PMD_SECT_VALID which is used for THP invalidation. Hence
>> it will not be there for pmd_present() check after pmdp_invalidate().
>>
>> A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD
>> entry during invalidation which can help pmd_present() return true and in
>> recognizing the fact that it still points to memory.
>>
>> This bit is transient. During the split process it will be overridden by a
>> page table page representing normal pages in place of erstwhile huge page.
>> Other pmdp_invalidate() callers always write a fresh PMD value on the entry
>> overriding this transient PMD_PRESENT_INVALID bit, which makes it safe.
>>
>> [1]: https://lkml.org/lkml/2018/10/17/231
>> [2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt
> 
> Hi Anshuman,
> 
> One query on this.  From my reading of the ARM ARM, bit 59 is not
> an ignored bit.  The exact requirements for hardware to be using
> it are a bit complex though.
> 
> It 'might' be safe to use it for this, but if so can we have a comment
> explaining why.  Also more than possible I'm misunderstanding things! 

We are using this bit 59 only when the entry is not active from MMU
perspective i.e PMD_SECT_VALID is clear.