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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 06/27/2019 09:01 PM, Zi Yan wrote:
> On 27 Jun 2019, at 8:48, Anshuman Khandual 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_mknotpresent -> 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().
>>
>> PTE_SPECIAL never gets used for PMD mapping i.e there is no pmd_special().
>> Hence this bit can be set on the PMD entry during invalidation which can
>> help in making pmd_present() return true and in recognizing the fact that
>> it still points to memory.
>>
>> This bit is transient. During the split is process it will be overridden
>> by a page table page representing the normal pages in place of erstwhile
>> huge page. Other pmdp_invalidate() callers always write a fresh PMD value
>> on the entry overriding this transient PTE_SPECIAL making it safe. In the
>> past former pmd_[mk]splitting() functions used PTE_SPECIAL.
>>
>> [1]: https://lkml.org/lkml/2018/10/17/231
> 
> Just want to point out that lkml.org link might not be stable.
> This one would be better: https://lore.kernel.org/linux-mm/20181017020930.GN30832@redhat.com/

Sure will update the link in the commit. Thanks !