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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