mm, thp: do not queue fully unmapped pages for deferred split

Adding fully unmapped pages into deferred split queue is not productive:
these pages are about to be freed or they are pinned and cannot be split
anyway.

Link: http://lkml.kernel.org/r/20190913091849.11151-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 72a3280..b3e3819 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1292,12 +1292,20 @@ static void page_remove_anon_compound_rmap(struct page *page)
        if (TestClearPageDoubleMap(page)) {
                /*
                 * Subpages can be mapped with PTEs too. Check how many of
-                * themi are still mapped.
+                * them are still mapped.
                 */
                for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
                        if (atomic_add_negative(-1, &page[i]._mapcount))
                                nr++;
                }
+
+               /*
+                * Queue the page for deferred split if at least one small
+                * page of the compound page is unmapped, but at least one
+                * small page is still mapped.
+                */
+               if (nr && nr < HPAGE_PMD_NR)
+                       deferred_split_huge_page(page);
        } else {
                nr = HPAGE_PMD_NR;
        }
@@ -1305,10 +1313,8 @@ static void page_remove_anon_compound_rmap(struct page *page)
        if (unlikely(PageMlocked(page)))
                clear_page_mlock(page);
 
-       if (nr) {
+       if (nr)
                __mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, -nr);
-               deferred_split_huge_page(page);
-       }
 }
 
 /**