Re: [PATCHv9 26/36] mm: rework mapcount accounting to enable 4k mapping of THPs

[Jerome Marchand]

On 08/03/2015 12:43 PM, Kirill A. Shutemov wrote:
> On Fri, Jul 31, 2015 at 05:04:18PM +0200, Jerome Marchand wrote:
>> On 07/20/2015 04:20 PM, Kirill A. Shutemov wrote:
>>> We're going to allow mapping of individual 4k pages of THP compound.
>>> It means we need to track mapcount on per small page basis.
>>>
>>> Straight-forward approach is to use ->_mapcount in all subpages to track
>>> how many time this subpage is mapped with PMDs or PTEs combined. But
>>> this is rather expensive: mapping or unmapping of a THP page with PMD
>>> would require HPAGE_PMD_NR atomic operations instead of single we have
>>> now.
>>>
>>> The idea is to store separately how many times the page was mapped as
>>> whole -- compound_mapcount. This frees up ->_mapcount in subpages to
>>> track PTE mapcount.
>>>
>>> We use the same approach as with compound page destructor and compound
>>> order to store compound_mapcount: use space in first tail page,
>>> ->mapping this time.
>>>
>>> Any time we map/unmap whole compound page (THP or hugetlb) -- we
>>> increment/decrement compound_mapcount. When we map part of compound page
>>> with PTE we operate on ->_mapcount of the subpage.
>>>
>>> page_mapcount() counts both: PTE and PMD mappings of the page.
>>>
>>> Basically, we have mapcount for a subpage spread over two counters.
>>> It makes tricky to detect when last mapcount for a page goes away.
>>>
>>> We introduced PageDoubleMap() for this. When we split THP PMD for the
>>> first time and there's other PMD mapping left we offset up ->_mapcount
>>> in all subpages by one and set PG_double_map on the compound page.
>>> These additional references go away with last compound_mapcount.
>>
>> So this stays even if all PTE mappings goes and the page is again mapped
>> only with PMD. I'm not sure how often that happen and if it's an issue
>> worth caring about.
> 
> We don't have a cheap way to detect this situation and it shouldn't
> happen often enough to care.
> 

I thought so.

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