lkml.org 
[lkml]   [2019]   [Jun]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics
From
Date


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 !

\
 
 \ /
  Last update: 2019-06-28 05:46    [W:0.127 / U:0.084 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site