[PATCH v4 03/12] mm/rmap: refactor some code around lazyfree folio unmapping

[Dev Jain]

For lazyfree folio unmapping, after clearing the ptes we must abort the
operation if the folio got dirtied or it has unexpected references.

Refactor this logic into a function which will return whether we need
to abort or not.

If we abort, we restore the ptes and bail out of try_to_unmap_one.
Otherwise adjust the rss stats of the mm and jump to a label.

Also rename that label from "discard" to "finish_unmap"; the former
is appropriate in the lazyfree context, but the code following the label
is executed for other successful unmap code paths too, so 'discard' does
not sound correct for them.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
 mm/rmap.c | 88 +++++++++++++++++++++++++++++++------------------------
 1 file changed, 50 insertions(+), 38 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 06ab1158d4cd1..12bbee57f20da 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2093,6 +2093,52 @@ static bool try_to_unmap_hugetlb_one(struct folio *folio,
 	return ret;
 }
 
+static inline bool ttu_lazyfree_folio(struct vm_area_struct *vma,
+		struct folio *folio)
+{
+	int ref_count, map_count;
+
+	/*
+	 * Synchronize with gup_pte_range():
+	 * - clear PTE; barrier; read refcount
+	 * - inc refcount; barrier; read PTE
+	 */
+	smp_mb();
+
+	ref_count = folio_ref_count(folio);
+	map_count = folio_mapcount(folio);
+
+	/*
+	 * Order reads for page refcount and dirty flag
+	 * (see comments in __remove_mapping()).
+	 */
+	smp_rmb();
+
+	if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+		/*
+		 * redirtied either using the page table or a previously
+		 * obtained GUP reference.
+		 */
+		folio_set_swapbacked(folio);
+		return false;
+	}
+
+	if (ref_count != 1 + map_count) {
+		/*
+		 * Additional reference. Could be a GUP reference or any
+		 * speculative reference. GUP users must mark the folio
+		 * dirty if there was a modification. This folio cannot be
+		 * reclaimed right now either way, so act just like nothing
+		 * happened.
+		 * We'll come back here later and detect if the folio was
+		 * dirtied when the additional reference is gone.
+		 */
+		return false;
+	}
+
+	return true;
+}
+
 /*
  * @arg: enum ttu_flags will be passed to this argument
  */
@@ -2279,47 +2325,13 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 
 			/* MADV_FREE page check */
 			if (!folio_test_swapbacked(folio)) {
-				int ref_count, map_count;
-
-				/*
-				 * Synchronize with gup_pte_range():
-				 * - clear PTE; barrier; read refcount
-				 * - inc refcount; barrier; read PTE
-				 */
-				smp_mb();
-
-				ref_count = folio_ref_count(folio);
-				map_count = folio_mapcount(folio);
-
-				/*
-				 * Order reads for page refcount and dirty flag
-				 * (see comments in __remove_mapping()).
-				 */
-				smp_rmb();
-
-				if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
-					/*
-					 * redirtied either using the page table or a previously
-					 * obtained GUP reference.
-					 */
-					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
-					folio_set_swapbacked(folio);
-					goto walk_abort;
-				} else if (ref_count != 1 + map_count) {
-					/*
-					 * Additional reference. Could be a GUP reference or any
-					 * speculative reference. GUP users must mark the folio
-					 * dirty if there was a modification. This folio cannot be
-					 * reclaimed right now either way, so act just like nothing
-					 * happened.
-					 * We'll come back here later and detect if the folio was
-					 * dirtied when the additional reference is gone.
-					 */
+				if (!ttu_lazyfree_folio(vma, folio)) {
 					set_ptes(mm, address, pvmw.pte, pteval, nr_pages);
 					goto walk_abort;
 				}
+
 				add_mm_counter(mm, MM_ANONPAGES, -nr_pages);
-				goto discard;
+				goto finish_unmap;
 			}
 
 			if (folio_dup_swap(folio, subpage) < 0) {
@@ -2382,7 +2394,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			 */
 			add_mm_counter(mm, mm_counter_file(folio), -nr_pages);
 		}
-discard:
+finish_unmap:
 		folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
 		if (vma->vm_flags & VM_LOCKED)
 			mlock_drain_local();
-- 
2.34.1