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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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.
> 
> 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.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

checkpatch fixlet:

diff --git a/mm/rmap.c b/mm/rmap.c
index bae27e4608b5..b8da0cdf50ae 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1073,8 +1073,11 @@ void do_page_add_anon_rmap(struct page *page,
 	if (PageTransCompound(page)) {
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
 		if (compound) {
+			atomic_t *mapcount;
+
 			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
-			first = atomic_inc_and_test(compound_mapcount_ptr(page));
+			mapcount = compound_mapcount_ptr(page);
+			first = atomic_inc_and_test(mapcount);
 		} else {
 			/* Anon THP always mapped first with PMD */
 			first = 0;

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