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. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>