f2fs: introduce F2FS_IOC_START_ATOMIC_REPLACE

introduce a new ioctl to replace the whole content of a file atomically,
which means it induces truncate and content update at the same time.
We can start it with F2FS_IOC_START_ATOMIC_REPLACE and complete it with
F2FS_IOC_COMMIT_ATOMIC_WRITE. Or abort it with
F2FS_IOC_ABORT_ATOMIC_WRITE.

Signed-off-by: Daeho Jeong <daehojeong@google.com>

fs/f2fs/data.c

@@ -3438,6 +3438,9 @@ static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi,
 	else if (*blk_addr != NULL_ADDR)
 		return 0;
 
+	if (is_inode_flag_set(inode, FI_ATOMIC_TRUNCATED))
+		goto reserve_block;
+
 	/* Look for the block in the original inode */
 	err = __find_data_block(inode, index, &ori_blk_addr);
 	if (err)

fs/f2fs/f2fs.h

@@ -764,6 +764,7 @@ enum {
 	FI_COMPRESS_RELEASED,	/* compressed blocks were released */
 	FI_ALIGNED_WRITE,	/* enable aligned write */
 	FI_COW_FILE,		/* indicate COW file */
+	FI_ATOMIC_TRUNCATED,	/* indicate truncated atomic write */
 	FI_MAX,			/* max flag, never be used */
 };
 

fs/f2fs/file.c

@@ -1982,7 +1982,7 @@ static int f2fs_ioc_getversion(struct file *filp, unsigned long arg)
 	return put_user(inode->i_generation, (int __user *)arg);
 }
 
-static int f2fs_ioc_start_atomic_write(struct file *filp)
+static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
 {
 	struct inode *inode = file_inode(filp);
 	struct user_namespace *mnt_userns = file_mnt_user_ns(filp);
@@ -2051,6 +2051,12 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
 
 	isize = i_size_read(inode);
 	fi->original_i_size = isize;
+	if (truncate) {
+		set_inode_flag(inode, FI_ATOMIC_TRUNCATED);
+		truncate_inode_pages_final(inode->i_mapping);
+		f2fs_i_size_write(inode, 0);
+		isize = 0;
+	}
 	f2fs_i_size_write(fi->cow_inode, isize);
 
 	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
@@ -4080,7 +4086,9 @@ static long __f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	case FS_IOC_GETVERSION:
 		return f2fs_ioc_getversion(filp, arg);
 	case F2FS_IOC_START_ATOMIC_WRITE:
-		return f2fs_ioc_start_atomic_write(filp);
+		return f2fs_ioc_start_atomic_write(filp, false);
+	case F2FS_IOC_START_ATOMIC_REPLACE:
+		return f2fs_ioc_start_atomic_write(filp, true);
 	case F2FS_IOC_COMMIT_ATOMIC_WRITE:
 		return f2fs_ioc_commit_atomic_write(filp);
 	case F2FS_IOC_ABORT_ATOMIC_WRITE:

fs/f2fs/segment.c

@@ -263,14 +263,26 @@ static void __complete_revoke_list(struct inode *inode, struct list_head *head,
 					bool revoke)
 {
 	struct revoke_entry *cur, *tmp;
+	pgoff_t start_index = 0;
+	bool truncate = is_inode_flag_set(inode, FI_ATOMIC_TRUNCATED);
 
 	list_for_each_entry_safe(cur, tmp, head, list) {
-		if (revoke)
+		if (revoke) {
 			__replace_atomic_write_block(inode, cur->index,
 						cur->old_addr, NULL, true);
+		} else if (truncate) {
+			f2fs_truncate_hole(inode, start_index, cur->index);
+			start_index = cur->index + 1;
+		}
+
 		list_del(&cur->list);
 		kmem_cache_free(revoke_entry_slab, cur);
 	}
+
+	if (!revoke && truncate) {
+		f2fs_do_truncate_blocks(inode, start_index * PAGE_SIZE, false);
+		clear_inode_flag(inode, FI_ATOMIC_TRUNCATED);
+	}
 }
 
 static int __f2fs_commit_atomic_write(struct inode *inode)

include/uapi/linux/f2fs.h

@@ -42,6 +42,8 @@
 						struct f2fs_comp_option)
 #define F2FS_IOC_DECOMPRESS_FILE	_IO(F2FS_IOCTL_MAGIC, 23)
 #define F2FS_IOC_COMPRESS_FILE		_IO(F2FS_IOCTL_MAGIC, 24)
+#define F2FS_IOC_START_TRUNC_ATOMIC_WRITE				\
+					_IO(F2FS_IOCTL_MAGIC, 25)
 
 /*
  * should be same as XFS_IOC_GOINGDOWN.