diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index 38ae34b7aaf4..f90f6ddd07a5 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -9,6 +9,7 @@
 #include <linux/mount.h>
 #include <linux/xattr.h>
 #include <linux/file.h>
+#include <linux/falloc.h>
 #include <trace/events/fscache.h>
 #include "internal.h"
 
@@ -134,55 +135,72 @@ struct cachefiles_object *cachefiles_grab_object(struct cachefiles_object *objec
 }
 
 /*
- * update the auxiliary data for an object object on disk
+ * Shorten the backing object to discard any dirty data and free up
+ * any unused granules.
  */
-static void cachefiles_update_object(struct cachefiles_object *object)
+static bool cachefiles_shorten_object(struct cachefiles_object *object,
+				      struct file *file, loff_t new_size)
 {
 	struct cachefiles_cache *cache = object->volume->cache;
-	const struct cred *saved_cred;
-	struct file *file = object->file;
-	loff_t object_size, i_size;
+	struct inode *inode = file_inode(file);
+	loff_t i_size, dio_size;
 	int ret;
 
-	_enter("{OBJ%x}", object->debug_id);
+	dio_size = round_up(new_size, CACHEFILES_DIO_BLOCK_SIZE);
+	i_size = i_size_read(inode);
 
-	cachefiles_begin_secure(cache, &saved_cred);
+	trace_cachefiles_trunc(object, inode, i_size, dio_size,
+			       cachefiles_trunc_shrink);
+	ret = vfs_truncate(&file->f_path, dio_size);
+	if (ret < 0) {
+		cachefiles_io_error_obj(object, "Trunc-to-size failed %d", ret);
+		cachefiles_remove_object_xattr(cache, file->f_path.dentry);
+		return false;
+	}
 
-	object_size = object->cookie->object_size;
-	i_size = i_size_read(file_inode(file));
-	if (i_size > object_size) {
-		_debug("trunc %llx -> %llx", i_size, object_size);
-		trace_cachefiles_trunc(object, file_inode(file),
-				       i_size, object_size,
-				       cachefiles_trunc_shrink);
-		ret = vfs_truncate(&file->f_path, object_size);
+	if (new_size < dio_size) {
+		trace_cachefiles_trunc(object, inode, dio_size, new_size,
+				       cachefiles_trunc_dio_adjust);
+		ret = vfs_fallocate(file, FALLOC_FL_ZERO_RANGE,
+				    new_size, dio_size);
 		if (ret < 0) {
-			cachefiles_io_error_obj(object, "Trunc-to-size failed");
+			cachefiles_io_error_obj(object, "Trunc-to-dio-size failed %d", ret);
 			cachefiles_remove_object_xattr(cache, file->f_path.dentry);
-			goto out;
-		}
-
-		object_size = round_up(object_size, CACHEFILES_DIO_BLOCK_SIZE);
-		i_size = i_size_read(file_inode(file));
-		_debug("trunc %llx -> %llx", i_size, object_size);
-		if (i_size < object_size) {
-			trace_cachefiles_trunc(object, file_inode(file),
-					       i_size, object_size,
-					       cachefiles_trunc_dio_adjust);
-			ret = vfs_truncate(&file->f_path, object_size);
-			if (ret < 0) {
-				cachefiles_io_error_obj(object, "Trunc-to-dio-size failed");
-				cachefiles_remove_object_xattr(cache, file->f_path.dentry);
-				goto out;
-			}
+			return false;
 		}
 	}
 
-	cachefiles_set_object_xattr(object);
+	return true;
+}
 
-out:
-	cachefiles_end_secure(cache, saved_cred);
-	_leave("");
+/*
+ * Resize the backing object.
+ */
+static void cachefiles_resize_cookie(struct netfs_cache_resources *cres,
+				     loff_t new_size)
+{
+	struct cachefiles_object *object = cachefiles_cres_object(cres);
+	struct cachefiles_cache *cache = object->volume->cache;
+	struct fscache_cookie *cookie = object->cookie;
+	const struct cred *saved_cred;
+	struct file *file = cachefiles_cres_file(cres);
+	loff_t old_size = cookie->object_size;
+
+	_enter("%llu->%llu", old_size, new_size);
+
+	if (new_size < old_size) {
+		cachefiles_begin_secure(cache, &saved_cred);
+		cachefiles_shorten_object(object, file, new_size);
+		cachefiles_end_secure(cache, saved_cred);
+		object->cookie->object_size = new_size;
+		return;
+	}
+
+	/* The file is being expanded.  We don't need to do anything
+	 * particularly.  cookie->initial_size doesn't change and so the point
+	 * at which we have to download before doesn't change.
+	 */
+	cookie->object_size = new_size;
 }
 
 /*
@@ -196,7 +214,7 @@ static void cachefiles_commit_object(struct cachefiles_object *object,
 	if (test_and_clear_bit(FSCACHE_COOKIE_NEEDS_UPDATE, &object->cookie->flags))
 		update = true;
 	if (update)
-		cachefiles_update_object(object);
+		cachefiles_set_object_xattr(object);
 
 	if (test_bit(CACHEFILES_OBJECT_USING_TMPFILE, &object->flags))
 		cachefiles_commit_tmpfile(cache, object);
@@ -440,5 +458,6 @@ const struct fscache_cache_ops cachefiles_cache_ops = {
 	.lookup_cookie		= cachefiles_lookup_cookie,
 	.withdraw_cookie	= cachefiles_withdraw_cookie,
 	.invalidate_cookie	= cachefiles_invalidate_cookie,
+	.resize_cookie		= cachefiles_resize_cookie,
 	.begin_operation	= cachefiles_begin_operation,
 };
diff --git a/fs/fscache/io.c b/fs/fscache/io.c
index ad9798f0c850..8b1a865a0847 100644
--- a/fs/fscache/io.c
+++ b/fs/fscache/io.c
@@ -253,3 +253,28 @@ int fscache_set_page_dirty(struct page *page, struct fscache_cookie *cookie)
 	return 1;
 }
 EXPORT_SYMBOL(fscache_set_page_dirty);
+
+/*
+ * Change the size of a backing object.
+ */
+void __fscache_resize_cookie(struct fscache_cookie *cookie, loff_t new_size)
+{
+	struct netfs_cache_resources cres;
+
+	trace_fscache_resize(cookie, new_size);
+	if (fscache_begin_operation(&cres, cookie, FSCACHE_WANT_WRITE,
+				    fscache_access_io_resize) == 0) {
+		fscache_stat(&fscache_n_resizes);
+		set_bit(FSCACHE_COOKIE_NEEDS_UPDATE, &cookie->flags);
+
+		/* We cannot defer a resize as we need to do it inside the
+		 * netfs's inode lock so that we're serialised with respect to
+		 * writes.
+		 */
+		cookie->volume->cache->ops->resize_cookie(&cres, new_size);
+		fscache_end_operation(&cres);
+	} else {
+		fscache_stat(&fscache_n_resizes_null);
+	}
+}
+EXPORT_SYMBOL(__fscache_resize_cookie);
diff --git a/fs/fscache/stats.c b/fs/fscache/stats.c
index 44b3b901e191..b1174d0dac3e 100644
--- a/fs/fscache/stats.c
+++ b/fs/fscache/stats.c
@@ -97,8 +97,10 @@ int fscache_stats_show(struct seq_file *m, void *v)
 	seq_printf(m, "Invals : n=%u\n",
 		   atomic_read(&fscache_n_invalidates));
 
-	seq_printf(m, "Updates: n=%u\n",
-		   atomic_read(&fscache_n_updates));
+	seq_printf(m, "Updates: n=%u rsz=%u rsn=%u\n",
+		   atomic_read(&fscache_n_updates),
+		   atomic_read(&fscache_n_resizes),
+		   atomic_read(&fscache_n_resizes_null));
 
 	seq_printf(m, "Relinqs: n=%u rtr=%u drop=%u\n",
 		   atomic_read(&fscache_n_relinquishes),
diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 22064611d182..2e7265e24df6 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -66,6 +66,10 @@ struct fscache_cache_ops {
 	/* Withdraw an object without any cookie access counts held */
 	void (*withdraw_cookie)(struct fscache_cookie *cookie);
 
+	/* Change the size of a data object */
+	void (*resize_cookie)(struct netfs_cache_resources *cres,
+			      loff_t new_size);
+
 	/* Invalidate an object */
 	bool (*invalidate_cookie)(struct fscache_cookie *cookie);
 
diff --git a/include/linux/fscache.h b/include/linux/fscache.h
index 08663ad7feed..8148193045cd 100644
--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -164,6 +164,7 @@ extern struct fscache_cookie *__fscache_acquire_cookie(
 extern void __fscache_use_cookie(struct fscache_cookie *, bool);
 extern void __fscache_unuse_cookie(struct fscache_cookie *, const void *, const loff_t *);
 extern void __fscache_relinquish_cookie(struct fscache_cookie *, bool);
+extern void __fscache_resize_cookie(struct fscache_cookie *, loff_t);
 extern void __fscache_invalidate(struct fscache_cookie *, const void *, loff_t, unsigned int);
 #ifdef FSCACHE_USE_NEW_IO_API
 extern int __fscache_begin_read_operation(struct netfs_cache_resources *, struct fscache_cookie *);
@@ -361,6 +362,23 @@ void fscache_update_cookie(struct fscache_cookie *cookie, const void *aux_data,
 		__fscache_update_cookie(cookie, aux_data, object_size);
 }
 
+/**
+ * fscache_resize_cookie - Request that a cache object be resized
+ * @cookie: The cookie representing the cache object
+ * @new_size: The new size of the object (may be NULL)
+ *
+ * Request that the size of an object be changed.
+ *
+ * See Documentation/filesystems/caching/netfs-api.txt for a complete
+ * description.
+ */
+static inline
+void fscache_resize_cookie(struct fscache_cookie *cookie, loff_t new_size)
+{
+	if (fscache_cookie_enabled(cookie))
+		__fscache_resize_cookie(cookie, new_size);
+}
+
 /**
  * fscache_pin_cookie - Pin a data-storage cache object in its cache
  * @cookie: The cookie representing the cache object
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index adde3bb61f0f..9554b43e6fdf 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -429,6 +429,29 @@ TRACE_EVENT(fscache_invalidate,
 		      __entry->cookie, __entry->new_size)
 	    );
 
+TRACE_EVENT(fscache_resize,
+	    TP_PROTO(struct fscache_cookie *cookie, loff_t new_size),
+
+	    TP_ARGS(cookie, new_size),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		cookie		)
+		    __field(loff_t,			old_size	)
+		    __field(loff_t,			new_size	)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->cookie	= cookie->debug_id;
+		    __entry->old_size	= cookie->object_size;
+		    __entry->new_size	= new_size;
+			   ),
+
+	    TP_printk("c=%08x os=%08llx sz=%08llx",
+		      __entry->cookie,
+		      __entry->old_size,
+		      __entry->new_size)
+	    );
+
 #endif /* _TRACE_FSCACHE_H */
 
 /* This part must be outside protection */
