I've been trying to understand whether we can simplify the mapcount handling for folios from the current situation with THPs. Let me quote the commit message from 53f9263baba6: > mm: rework mapcount accounting to enable 4k mapping of THPs > > 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. > > This approach provides a way to detect when last mapcount goes away on > per small page basis without introducing new overhead for most common > cases. What breaks if we simply track any mapping (whether by PMD or PTE) as an increment to the head page (aka folio's) refcount? Essentially, we make the head mapcount 'the number of VMAs which contain a reference to any page in this folio'. We can remove PageDoubleMap. The tail refcounts will all be 0. If it's useful, we could introduce a 'partial_mapcount' which would be <= mapcount (but I don't know if it's useful). Splitting a PMD would not change ->_mapcount. Splitting the folio already causes the folio to be unmapped, so page faults will naturally re-increment ->_mapcount of each subpage. We might need some additional logic to treat a large folio (aka compound page) as a single unit; that is, when we fault on one page, we place entries for all pages in this folio (that fit ...) into the page tables, so that we only account it once, even if it's not compatible with using a PMD.
