fuse: add flag to turn on big writes

Prior to 2.6.26 fuse only supported single page write requests. In theory all
fuse filesystem should be able support bigger than 4k writes, as there's
nothing in the API to prevent it. Unfortunately there's a known case in
NTFS-3G where big writes cause filesystem corruption. There could also be
other filesystems, where the lack of testing with big write requests would
result in bugs.

To prevent such problems on a kernel upgrade, disable big writes by default,
but let filesystems set a flag to turn it on.

Signed-off-by: Miklos Szeredi <[e-mail]>
Cc: Szabolcs Szakacsits <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fuse/file.c

···
 		if (offset == PAGE_CACHE_SIZE)
 			offset = 0;
 
+		if (!fc->big_writes)
+			break;
 	} while (iov_iter_count(ii) && count < fc->max_write &&
 		 req->num_pages < FUSE_MAX_PAGES_PER_REQ && offset == 0);
 

fs/fuse/fuse_i.h

···
 	/** Is bmap not implemented by fs? */
 	unsigned no_bmap : 1;
 
+	/** Do multi-page cached writes */
+	unsigned big_writes : 1;
+
 	/** The number of requests waiting for completion */
 	atomic_t num_waiting;
 

fs/fuse/inode.c

···
 				fc->no_lock = 1;
 			if (arg->flags & FUSE_ATOMIC_O_TRUNC)
 				fc->atomic_o_trunc = 1;
+			if (arg->flags & FUSE_BIG_WRITES)
+				fc->big_writes = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_CACHE_SIZE;
 			fc->no_lock = 1;
···
 	arg->major = FUSE_KERNEL_VERSION;
 	arg->minor = FUSE_KERNEL_MINOR_VERSION;
 	arg->max_readahead = fc->bdi.ra_pages * PAGE_CACHE_SIZE;
-	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_ATOMIC_O_TRUNC;
+	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_ATOMIC_O_TRUNC |
+		FUSE_BIG_WRITES;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);

include/linux/fuse.h

···
 #define FUSE_POSIX_LOCKS	(1 << 1)
 #define FUSE_FILE_OPS		(1 << 2)
 #define FUSE_ATOMIC_O_TRUNC	(1 << 3)
+#define FUSE_BIG_WRITES		(1 << 5)
 
 /**
  * Release flags