[RFC v2 3/5] mm/migrate: refactor add_page_for_migration for code re-use

[Gregory Price]

add_page_for_migration presently does two actions:
  1) validates the page is present and migratable
  2) isolates the page from LRU and puts it into the migration list

Break add_page_for_migration into 2 functions:
  add_page_for_migration - isolate the page from LUR and add to list
  add_virt_page_for_migration - validate the page and call the above

add_page_for_migration does not require the mm_struct and so can be
re-used for a physical addressing version of move_pages

Signed-off-by: Gregory Price <gregory.price@xxxxxxxxxxxx>
---
 mm/migrate.c | 83 +++++++++++++++++++++++++++++++---------------------
 1 file changed, 50 insertions(+), 33 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index dbe436163d65..1123d841a7f1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2042,52 +2042,33 @@ static int do_move_pages_to_node(struct list_head *pagelist, int node)
 }
 
 /*
- * Resolves the given address to a struct page, isolates it from the LRU and
- * puts it to the given pagelist.
+ * Isolates the page from the LRU and puts it into the given pagelist
  * Returns:
  *     errno - if the page cannot be found/isolated
  *     0 - when it doesn't have to be migrated because it is already on the
  *         target node
  *     1 - when it has been queued
  */
-static int add_page_for_migration(struct mm_struct *mm, const void __user *p,
-		int node, struct list_head *pagelist, bool migrate_all)
+static int add_page_for_migration(struct page *page, int node,
+		struct list_head *pagelist, bool migrate_all)
 {
-	struct vm_area_struct *vma;
-	unsigned long addr;
-	struct page *page;
 	int err;
 	bool isolated;
 
-	mmap_read_lock(mm);
-	addr = (unsigned long)untagged_addr_remote(mm, p);
-
-	err = -EFAULT;
-	vma = vma_lookup(mm, addr);
-	if (!vma || !vma_migratable(vma))
-		goto out;
-
-	/* FOLL_DUMP to ignore special (like zero) pages */
-	page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
-
-	err = PTR_ERR(page);
-	if (IS_ERR(page))
-		goto out;
-
 	err = -ENOENT;
 	if (!page)
 		goto out;
 
 	if (is_zone_device_page(page))
-		goto out_putpage;
+		goto out;
 
 	err = 0;
 	if (page_to_nid(page) == node)
-		goto out_putpage;
+		goto out;
 
 	err = -EACCES;
 	if (page_mapcount(page) > 1 && !migrate_all)
-		goto out_putpage;
+		goto out;
 
 	if (PageHuge(page)) {
 		if (PageHead(page)) {
@@ -2101,7 +2082,7 @@ static int add_page_for_migration(struct mm_struct *mm, const void __user *p,
 		isolated = isolate_lru_page(head);
 		if (!isolated) {
 			err = -EBUSY;
-			goto out_putpage;
+			goto out;
 		}
 
 		err = 1;
@@ -2110,12 +2091,48 @@ static int add_page_for_migration(struct mm_struct *mm, const void __user *p,
 			NR_ISOLATED_ANON + page_is_file_lru(head),
 			thp_nr_pages(head));
 	}
-out_putpage:
-	/*
-	 * Either remove the duplicate refcount from
-	 * isolate_lru_page() or drop the page ref if it was
-	 * not isolated.
-	 */
+out:
+	return err;
+}
+
+/*
+ * Resolves the given address to a struct page, isolates it from the LRU and
+ * puts it to the given pagelist.
+ * Returns:
+ *     errno - if the page cannot be found/isolated
+ *     0 - when it doesn't have to be migrated because it is already on the
+ *         target node
+ *     1 - when it has been queued
+ */
+static int add_virt_page_for_migration(struct mm_struct *mm,
+		const void __user *p, int node, struct list_head *pagelist,
+		bool migrate_all)
+{
+	struct vm_area_struct *vma;
+	unsigned long addr;
+	struct page *page;
+	int err = -EFAULT;
+
+	mmap_read_lock(mm);
+	addr = (unsigned long)untagged_addr_remote(mm, p);
+
+	vma = vma_lookup(mm, addr);
+	if (!vma || !vma_migratable(vma))
+		goto out;
+
+	/* FOLL_DUMP to ignore special (like zero) pages */
+	page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
+
+	err = PTR_ERR(page);
+	if (IS_ERR(page))
+		goto out;
+
+	err = -ENOENT;
+	if (!page)
+		goto out;
+
+	err = add_page_for_migration(page, node, pagelist, migrate_all);
+
 	put_page(page);
 out:
 	mmap_read_unlock(mm);
@@ -2211,7 +2228,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
 		 * Errors in the page lookup or isolation are not fatal and we simply
 		 * report them via status
 		 */
-		err = add_page_for_migration(mm, p, current_node, &pagelist,
+		err = add_virt_page_for_migration(mm, p, current_node, &pagelist,
 					     flags & MPOL_MF_MOVE_ALL);
 
 		if (err > 0) {
-- 
2.39.1