f2fs: support volatile operations for transient data

This patch adds support for volatile writes which keep data pages in memory
until f2fs_evict_inode is called by iput.

For instance, we can use this feature for the sqlite database as follows.
While supporting atomic writes for main database file, we can keep its journal
data temporarily in the page cache by the following sequence.

1. open
 -> ioctl(F2FS_IOC_START_VOLATILE_WRITE);
2. writes
 : keep all the data in the page cache.
3. flush to the database file with atomic writes
  a. ioctl(F2FS_IOC_START_ATOMIC_WRITE);
  b. writes
  c. ioctl(F2FS_IOC_COMMIT_ATOMIC_WRITE);
4. close
 -> drop the cached data

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 3708b80..8e68bb6 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -885,6 +885,9 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
+	if (f2fs_is_volatile_file(inode))
+		return 0;
+
 	ret = mnt_want_write_file(filp);
 	if (ret)
 		return ret;
@@ -897,6 +900,17 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
 	return ret;
 }
 
+static int f2fs_ioc_start_volatile_write(struct file *filp)
+{
+	struct inode *inode = file_inode(filp);
+
+	if (!inode_owner_or_capable(inode))
+		return -EACCES;
+
+	set_inode_flag(F2FS_I(inode), FI_VOLATILE_FILE);
+	return 0;
+}
+
 static int f2fs_ioc_fitrim(struct file *filp, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -938,6 +952,8 @@ long f2fs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		return f2fs_ioc_start_atomic_write(filp);
 	case F2FS_IOC_COMMIT_ATOMIC_WRITE:
 		return f2fs_ioc_commit_atomic_write(filp);
+	case F2FS_IOC_START_VOLATILE_WRITE:
+		return f2fs_ioc_start_volatile_write(filp);
 	case FITRIM:
 		return f2fs_ioc_fitrim(filp, arg);
 	default: