Btrfs: transaction ioctls

These ioctls let a user application hold a transaction open while it
performs a series of operations.  A final ioctl does a sync on the fs
(closing the current transaction).  This is the main requirement for
Ceph's OSD to be able to keep the data it's storing in a btrfs volume
consistent, and AFAICS it works just fine.  The application would do
something like

	fd = ::open("some/file", O_RDONLY);
	::ioctl(fd, BTRFS_IOC_TRANS_START);
	/* do a bunch of stuff */
	::ioctl(fd, BTRFS_IOC_TRANS_END);
or just
	::close(fd);

And to ensure it commits to disk,

	::ioctl(fd, BTRFS_IOC_SYNC);

When a transaction is held open, the trans_handle is attached to the
struct file (via private_data) so that it will get cleaned up if the
process dies unexpectedly.  A held transaction is also ended on fsync() to
avoid a deadlock.

A misbehaving application could also deliberately hold a transaction open,
effectively locking up the FS, so it may make sense to restrict something
like this to root or something.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

5 files changed, 83 insertions, 2 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index ad4eacca7f59..1dcf4fb5b688 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1575,6 +1575,7 @@ struct inode *btrfs_alloc_inode(struct super_block *sb);
 void btrfs_destroy_inode(struct inode *inode);
 int btrfs_init_cachep(void);
 void btrfs_destroy_cachep(void);
+long btrfs_ioctl_trans_end(struct file *file);
 long btrfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 struct inode *btrfs_iget_locked(struct super_block *s, u64 objectid,
                                 struct btrfs_root *root);
@@ -1595,6 +1596,8 @@ extern struct file_operations btrfs_file_operations;
 int btrfs_drop_extents(struct btrfs_trans_handle *trans,
                        struct btrfs_root *root, struct inode *inode,
                        u64 start, u64 end, u64 inline_limit, u64 *hint_block);
+int btrfs_release_file(struct inode *inode, struct file *file);
+
 /* tree-defrag.c */
 int btrfs_defrag_leaves(struct btrfs_trans_handle *trans,
                         struct btrfs_root *root, int cache_only);
@@ -1615,4 +1618,5 @@ int btrfs_delete_xattrs(struct btrfs_trans_handle *trans,
 u64 btrfs_parse_size(char *str);
 int btrfs_parse_options(char *options, struct btrfs_root *root,
                         char **subvol_name);
+int btrfs_sync_fs(struct super_block *sb, int wait);
 #endif
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index c4fa4664a456..73c6d085bd90 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -978,9 +978,11 @@ out_nolock:
         return num_written ? num_written : err;
 }
 
-static int btrfs_release_file (struct inode * inode, struct file * filp)
+int btrfs_release_file(struct inode * inode, struct file * filp)
 {
         btrfs_del_ordered_inode(inode);
+        if (filp->private_data)
+                btrfs_ioctl_trans_end(filp);
         return 0;
 }
 
@@ -1011,6 +1013,9 @@ static int btrfs_sync_file(struct file *file,
         /*
          * ok we haven't committed the transaction yet, lets do a commit
          */
+        if (file->private_data)
+                btrfs_ioctl_trans_end(file);
+
         trans = btrfs_start_transaction(root, 1);
         if (!trans) {
                 ret = -ENOMEM;
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 31aa4ba06fce..0f14697becef 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3336,6 +3336,61 @@ out_fput:
         return ret;
 }
 
+/*
+ * there are many ways the trans_start and trans_end ioctls can lead
+ * to deadlocks.  They should only be used by applications that
+ * basically own the machine, and have a very in depth understanding
+ * of all the possible deadlocks and enospc problems.
+ */
+long btrfs_ioctl_trans_start(struct file *file)
+{
+        struct inode *inode = fdentry(file)->d_inode;
+        struct btrfs_root *root = BTRFS_I(inode)->root;
+        struct btrfs_trans_handle *trans;
+        int ret = 0;
+
+        mutex_lock(&root->fs_info->fs_mutex);
+        if (file->private_data) {
+                ret = -EINPROGRESS;
+                goto out;
+        }
+        trans = btrfs_start_transaction(root, 0);
+        if (trans)
+                file->private_data = trans;
+        else
+                ret = -ENOMEM;
+        /*printk(KERN_INFO "btrfs_ioctl_trans_start on %p\n", file);*/
+out:
+        mutex_unlock(&root->fs_info->fs_mutex);
+        return ret;
+}
+
+/*
+ * there are many ways the trans_start and trans_end ioctls can lead
+ * to deadlocks.  They should only be used by applications that
+ * basically own the machine, and have a very in depth understanding
+ * of all the possible deadlocks and enospc problems.
+ */
+long btrfs_ioctl_trans_end(struct file *file)
+{
+        struct inode *inode = fdentry(file)->d_inode;
+        struct btrfs_root *root = BTRFS_I(inode)->root;
+        struct btrfs_trans_handle *trans;
+        int ret = 0;
+
+        mutex_lock(&root->fs_info->fs_mutex);
+        trans = file->private_data;
+        if (!trans) {
+                ret = -EINVAL;
+                goto out;
+        }
+        btrfs_end_transaction(trans, root);
+        file->private_data = 0;
+out:
+        mutex_unlock(&root->fs_info->fs_mutex);
+        return ret;
+}
+
 long btrfs_ioctl(struct file *file, unsigned int
                 cmd, unsigned long arg)
 {
@@ -3356,6 +3411,13 @@ long btrfs_ioctl(struct file *file, unsigned int
                 return btrfs_balance(root->fs_info->dev_root);
         case BTRFS_IOC_CLONE:
                 return btrfs_ioctl_clone(file, arg);
+        case BTRFS_IOC_TRANS_START:
+                return btrfs_ioctl_trans_start(file);
+        case BTRFS_IOC_TRANS_END:
+                return btrfs_ioctl_trans_end(file);
+        case BTRFS_IOC_SYNC:
+                btrfs_sync_fs(file->f_dentry->d_sb, 1);
+                return 0;
         }
 
         return -ENOTTY;
@@ -3679,6 +3741,7 @@ static struct file_operations btrfs_dir_file_operations = {
 #ifdef CONFIG_COMPAT
         .compat_ioctl   = btrfs_ioctl,
 #endif
+        .release        = btrfs_release_file,
 };
 
 static struct extent_io_ops btrfs_extent_io_ops = {
diff --git a/fs/btrfs/ioctl.h b/fs/btrfs/ioctl.h
index b0e73f51d636..85ed35a775b1 100644
--- a/fs/btrfs/ioctl.h
+++ b/fs/btrfs/ioctl.h
@@ -36,6 +36,14 @@ struct btrfs_ioctl_vol_args {
                                    struct btrfs_ioctl_vol_args)
 #define BTRFS_IOC_SCAN_DEV _IOW(BTRFS_IOCTL_MAGIC, 4, \
                                    struct btrfs_ioctl_vol_args)
+/* trans start and trans end are dangerous, and only for
+ * use by applications that know how to avoid the
+ * resulting deadlocks
+ */
+#define BTRFS_IOC_TRANS_START  _IO(BTRFS_IOCTL_MAGIC, 6)
+#define BTRFS_IOC_TRANS_END    _IO(BTRFS_IOCTL_MAGIC, 7)
+#define BTRFS_IOC_SYNC         _IO(BTRFS_IOCTL_MAGIC, 8)
+
 #define BTRFS_IOC_CLONE        _IOW(BTRFS_IOCTL_MAGIC, 9, int)
 #define BTRFS_IOC_ADD_DEV _IOW(BTRFS_IOCTL_MAGIC, 10, \
                                    struct btrfs_ioctl_vol_args)
@@ -43,4 +51,5 @@ struct btrfs_ioctl_vol_args {
                                    struct btrfs_ioctl_vol_args)
 #define BTRFS_IOC_BALANCE _IOW(BTRFS_IOCTL_MAGIC, 12, \
                                    struct btrfs_ioctl_vol_args)
+
 #endif
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 77f44494e229..39bb86945ed0 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -293,7 +293,7 @@ fail_close:
         return err;
 }
 
-static int btrfs_sync_fs(struct super_block *sb, int wait)
+int btrfs_sync_fs(struct super_block *sb, int wait)
 {
         struct btrfs_trans_handle *trans;
         struct btrfs_root *root;