mm: refactor check_and_migrate_movable_pages

Remove up to two levels of indentation by using continue statements
and move variables to local scope where possible.

Link: https://lkml.kernel.org/r/20220210072828.2930359-11-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Cc: Christian Knig <christian.koenig@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Logan Gunthorpe <logang@deltatee.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

mm/gup.c

@@ -1841,32 +1841,31 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
 					    struct page **pages,
 					    unsigned int gup_flags)
 {
-	unsigned long i;
-	unsigned long isolation_error_count = 0;
-	bool drain_allow = true;
-	LIST_HEAD(movable_page_list);
-	long ret = 0;
+	unsigned long isolation_error_count = 0, i;
 	struct page *prev_head = NULL;
-	struct page *head;
-	struct migration_target_control mtc = {
-		.nid = NUMA_NO_NODE,
-		.gfp_mask = GFP_USER | __GFP_NOWARN,
-	};
+	LIST_HEAD(movable_page_list);
+	bool drain_allow = true;
+	int ret = 0;
 
 	for (i = 0; i < nr_pages; i++) {
-		head = compound_head(pages[i]);
+		struct page *head = compound_head(pages[i]);
+
 		if (head == prev_head)
 			continue;
 		prev_head = head;
+
+		if (is_pinnable_page(head))
+			continue;
+
 		/*
-		 * If we get a movable page, since we are going to be pinning
-		 * these entries, try to move them out if possible.
+		 * Try to move out any movable page before pinning the range.
 		 */
-		if (!is_pinnable_page(head)) {
 		if (PageHuge(head)) {
 			if (!isolate_huge_page(head, &movable_page_list))
 				isolation_error_count++;
-			} else {
+			continue;
+		}
+
 		if (!PageLRU(head) && drain_allow) {
 			lru_add_drain_all();
 			drain_allow = false;
@@ -1878,35 +1877,43 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
 		}
 		list_add_tail(&head->lru, &movable_page_list);
 		mod_node_page_state(page_pgdat(head),
-						    NR_ISOLATED_ANON +
-						    page_is_file_lru(head),
+				    NR_ISOLATED_ANON + page_is_file_lru(head),
 				    thp_nr_pages(head));
 	}
-		}
-	}
+
+	if (!list_empty(&movable_page_list) || isolation_error_count)
+		goto unpin_pages;
 
 	/*
 	 * If list is empty, and no isolation errors, means that all pages are
 	 * in the correct zone.
 	 */
-	if (list_empty(&movable_page_list) && !isolation_error_count)
 	return nr_pages;
 
+unpin_pages:
 	if (gup_flags & FOLL_PIN) {
 		unpin_user_pages(pages, nr_pages);
 	} else {
 		for (i = 0; i < nr_pages; i++)
 			put_page(pages[i]);
 	}
+
 	if (!list_empty(&movable_page_list)) {
+		struct migration_target_control mtc = {
+			.nid = NUMA_NO_NODE,
+			.gfp_mask = GFP_USER | __GFP_NOWARN,
+		};
+
 		ret = migrate_pages(&movable_page_list, alloc_migration_target,
 				    NULL, (unsigned long)&mtc, MIGRATE_SYNC,
 				    MR_LONGTERM_PIN, NULL);
-		if (ret && !list_empty(&movable_page_list))
-			putback_movable_pages(&movable_page_list);
+		if (ret > 0) /* number of pages not migrated */
+			ret = -ENOMEM;
 	}
 
-	return ret > 0 ? -ENOMEM : ret;
+	if (ret && !list_empty(&movable_page_list))
+		putback_movable_pages(&movable_page_list);
+	return ret;
 }
 #else
 static long check_and_migrate_movable_pages(unsigned long nr_pages,