[PATCH v1 03/18] mm/rmap: add fast-path for small folios when adding/removing/duplicating

[David Hildenbrand]

Let's add a fast-path for small folios to all relevant rmap functions.
Note that only RMAP_LEVEL_PTE applies.

This is a preparation for tracking the mapcount of large folios in a
single value.

Signed-off-by: David Hildenbrand <da...@redhat.com>
---
 include/linux/rmap.h | 13 +++++++++++++
 mm/rmap.c            | 26 ++++++++++++++++----------
 2 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 9549d78928bb..327f1ca5a487 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -322,6 +322,11 @@ static __always_inline void __folio_dup_file_rmap(struct 
folio *folio,
 
        switch (level) {
        case RMAP_LEVEL_PTE:
+               if (!folio_test_large(folio)) {
+                       atomic_inc(&page->_mapcount);
+                       break;
+               }
+
                do {
                        atomic_inc(&page->_mapcount);
                } while (page++, --nr_pages > 0);
@@ -405,6 +410,14 @@ static __always_inline int 
__folio_try_dup_anon_rmap(struct folio *folio,
                                if (PageAnonExclusive(page + i))
                                        return -EBUSY;
                }
+
+               if (!folio_test_large(folio)) {
+                       if (PageAnonExclusive(page))
+                               ClearPageAnonExclusive(page);
+                       atomic_inc(&page->_mapcount);
+                       break;
+               }
+
                do {
                        if (PageAnonExclusive(page))
                                ClearPageAnonExclusive(page);
diff --git a/mm/rmap.c b/mm/rmap.c
index 56b313aa2ebf..4bde6d60db6c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1172,15 +1172,18 @@ static __always_inline unsigned int 
__folio_add_rmap(struct folio *folio,
 
        switch (level) {
        case RMAP_LEVEL_PTE:
+               if (!folio_test_large(folio)) {
+                       nr = atomic_inc_and_test(&page->_mapcount);
+                       break;
+               }
+
                do {
                        first = atomic_inc_and_test(&page->_mapcount);
-                       if (first && folio_test_large(folio)) {
+                       if (first) {
                                first = atomic_inc_return_relaxed(mapped);
-                               first = (first < ENTIRELY_MAPPED);
+                               if (first < ENTIRELY_MAPPED)
+                                       nr++;
                        }
-
-                       if (first)
-                               nr++;
                } while (page++, --nr_pages > 0);
                break;
        case RMAP_LEVEL_PMD:
@@ -1514,15 +1517,18 @@ static __always_inline void __folio_remove_rmap(struct 
folio *folio,
 
        switch (level) {
        case RMAP_LEVEL_PTE:
+               if (!folio_test_large(folio)) {
+                       nr = atomic_add_negative(-1, &page->_mapcount);
+                       break;
+               }
+
                do {
                        last = atomic_add_negative(-1, &page->_mapcount);
-                       if (last && folio_test_large(folio)) {
+                       if (last) {
                                last = atomic_dec_return_relaxed(mapped);
-                               last = (last < ENTIRELY_MAPPED);
+                               if (last < ENTIRELY_MAPPED)
+                                       nr++;
                        }
-
-                       if (last)
-                               nr++;
                } while (page++, --nr_pages > 0);
                break;
        case RMAP_LEVEL_PMD:
-- 
2.44.0