mm/filemap: convert filemap_update_page to return an errno

Use AOP_TRUNCATED_PAGE to indicate that no error occurred, but the page we
looked up is no longer valid.  In this case, the reference to the page
will have been removed; if we hit any other error, the caller will release
the reference.

Link: https://lkml.kernel.org/r/20210122160140.223228-12-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -2233,24 +2233,21 @@ static int filemap_read_page(struct file *file, struct address_space *mapping,
 	return error;
 }
 
-static struct page *filemap_update_page(struct kiocb *iocb, struct file *filp,
-		struct iov_iter *iter, struct page *page, loff_t pos,
-		loff_t count)
+static int filemap_update_page(struct kiocb *iocb,
+		struct address_space *mapping, struct iov_iter *iter,
+		struct page *page, loff_t pos, loff_t count)
 {
-	struct address_space *mapping = filp->f_mapping;
 	struct inode *inode = mapping->host;
 	int error;
 
 	if (iocb->ki_flags & IOCB_WAITQ) {
 		error = lock_page_async(page, iocb->ki_waitq);
-		if (error) {
-			put_page(page);
-			return ERR_PTR(error);
-		}
+		if (error)
+			return error;
 	} else {
 		if (!trylock_page(page)) {
 			put_and_wait_on_page_locked(page, TASK_KILLABLE);
-			return NULL;
+			return AOP_TRUNCATED_PAGE;
 		}
 	}
 
@@ -2269,25 +2266,21 @@ static struct page *filemap_update_page(struct kiocb *iocb, struct file *filp,
 		goto readpage;
 uptodate:
 	unlock_page(page);
-	return page;
+	return 0;
 
 readpage:
 	if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT | IOCB_WAITQ)) {
 		unlock_page(page);
-		put_page(page);
-		return ERR_PTR(-EAGAIN);
+		return -EAGAIN;
 	}
 	error = filemap_read_page(iocb->ki_filp, mapping, page);
-	if (!error)
-		return page;
-	put_page(page);
 	if (error == AOP_TRUNCATED_PAGE)
-		return NULL;
-	return ERR_PTR(error);
+		put_page(page);
+	return error;
 truncated:
 	unlock_page(page);
 	put_page(page);
-	return NULL;
+	return AOP_TRUNCATED_PAGE;
 }
 
 static int filemap_create_page(struct file *file,
@@ -2381,11 +2374,12 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 				goto err;
 			}
 
-			page = filemap_update_page(iocb, filp, iter, page,
+			err = filemap_update_page(iocb, mapping, iter, page,
 					pg_pos, pg_count);
-			if (IS_ERR_OR_NULL(page)) {
+			if (err) {
+				if (err < 0)
+					put_page(page);
 				pvec->nr--;
-				err = PTR_ERR_OR_ZERO(page);
 			}
 		}
 	}
@@ -2393,6 +2387,8 @@ static int filemap_get_pages(struct kiocb *iocb, struct iov_iter *iter,
 err:
 	if (likely(pvec->nr))
 		return 0;
+	if (err == AOP_TRUNCATED_PAGE)
+		goto find_page;
 	if (err)
 		return err;
 	/*