[RFC PATCH] overlayfs: Provide a mount option "nosync" to skip sync

[Vivek Goyal <vgoyal@redhat.com>]

Container folks are complaining that dnf/yum issues too many sync while
installing packages and this slows down the image build. Build
requirement is such that they don't care if a node goes down while
build was still going on. In that case, they will simply throw away
unfinished layer and start new build. So they don't care about syncing
intermediate state to the disk and hence don't want to pay the price
associated with sync.

So they are asking for an option where they can disable sync on overlay
mount point completely and user space will do sync management on upper
layer as needed.

They primarily seem to have two use cases.

- For building images, they will mount overlay with nosync and then sync
  upper layer after unmounting overlay and reuse upper as lower for next
  layer.

- For running containers, they don't seem to care about syncing upper
  layer because if node goes down, they will simply throw away upper
  layer and create a fresh one.

So this patch provides a mount option "nosync" which disables all forms
of sync. Now it is caller's responsibility to manage sync of upper layer
before it is reused again.

I am seeing roughly 20% speed up in my VM where I am just installing
emacs in an image. Installation time drops from 31 seconds to 25 seconds
when nosync option is used. This is for the case of building on top
of an image where all packages are already cached. That way I take
out the network operations latency out of the measurement.

Giuseppe is also looking to cut down on number of iops done on the
disk. He is complaining that often in cloud their VMs are throttled
if they cross the limit. This option can help them where they reduce
number of iops (by cutting down on frequent sync and writebacks).

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 Documentation/filesystems/overlayfs.rst | 20 ++++++++++++++++++++
 fs/overlayfs/copy_up.c                  | 12 ++++++++----
 fs/overlayfs/file.c                     | 11 ++++++++++-
 fs/overlayfs/ovl_entry.h                |  1 +
 fs/overlayfs/readdir.c                  |  3 +++
 fs/overlayfs/super.c                    | 23 ++++++++++++++++++++---
 6 files changed, 62 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 660dbaf0b9b8..0a42f26a3f0c 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -563,6 +563,26 @@ This verification may cause significant overhead in some cases.
 Note: the mount options index=off,nfs_export=on are conflicting and will
 result in an error.
 
+Disable sync
+------------
+By default, overlay skips sync on files residing on a lower layer.  It
+is possible to skip sync operations for files on the upper layer as well
+with the 'nosync' mount option. This option disables all forms of sync
+from overlay, including the one done at umount/remount and it is
+user's responsibility to sync upper layer on the file system it
+is residing.
+
+With this option, data loss will happen if overlayfs upper layer is
+not synced. So use this option very carefully. This is only for the
+use cases where users discard upper layer if they could not sync it
+successfully.
+
+Typically workflow will be.
+
+- mount overlay
+- Do bunch of operations
+- unmount overlay
+- sync filesystem container upper layer
 
 Testsuite
 ---------
diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 79dd052c7dbf..5431a89bbd8a 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -128,7 +128,8 @@ int ovl_copy_xattr(struct dentry *old, struct dentry *new)
 	return error;
 }
 
-static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
+static int ovl_copy_up_data(struct ovl_fs *ofs, struct path *old,
+			    struct path *new, loff_t len)
 {
 	struct file *old_file;
 	struct file *new_file;
@@ -218,7 +219,7 @@ static int ovl_copy_up_data(struct path *old, struct path *new, loff_t len)
 		len -= bytes;
 	}
 out:
-	if (!error)
+	if (!error && !ofs->config.nosync)
 		error = vfs_fsync(new_file, 0);
 	fput(new_file);
 out_fput:
@@ -484,6 +485,7 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
 
 static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 {
+	struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
 	int err;
 
 	/*
@@ -499,7 +501,8 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 		upperpath.dentry = temp;
 
 		ovl_path_lowerdata(c->dentry, &datapath);
-		err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
+		err = ovl_copy_up_data(ofs, &datapath, &upperpath,
+				       c->stat.size);
 		if (err)
 			return err;
 	}
@@ -784,6 +787,7 @@ static bool ovl_need_meta_copy_up(struct dentry *dentry, umode_t mode,
 /* Copy up data of an inode which was copied up metadata only in the past. */
 static int ovl_copy_up_meta_inode_data(struct ovl_copy_up_ctx *c)
 {
+	struct ovl_fs *ofs = OVL_FS(c->dentry->d_sb);
 	struct path upperpath, datapath;
 	int err;
 	char *capability = NULL;
@@ -804,7 +808,7 @@ static int ovl_copy_up_meta_inode_data(struct ovl_copy_up_ctx *c)
 			goto out;
 	}
 
-	err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
+	err = ovl_copy_up_data(ofs, &datapath, &upperpath, c->stat.size);
 	if (err)
 		goto out_free;
 
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 01820e654a21..a361890a8d05 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -329,6 +329,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	struct fd real;
 	const struct cred *old_cred;
 	ssize_t ret;
+	int ifl = iocb->ki_flags;
 
 	if (!iov_iter_count(iter))
 		return 0;
@@ -344,11 +345,14 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	if (ret)
 		goto out_unlock;
 
+	if (OVL_FS(inode->i_sb)->config.nosync)
+		ifl &= ~(IOCB_DSYNC | IOCB_SYNC);
+
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	if (is_sync_kiocb(iocb)) {
 		file_start_write(real.file);
 		ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
-				     ovl_iocb_to_rwf(iocb->ki_flags));
+				     ovl_iocb_to_rwf(ifl));
 		file_end_write(real.file);
 		/* Update size */
 		ovl_copyattr(ovl_inode_real(inode), inode);
@@ -368,6 +372,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		real.flags = 0;
 		aio_req->orig_iocb = iocb;
 		kiocb_clone(&aio_req->iocb, iocb, real.file);
+		aio_req->iocb.ki_flags = ifl;
 		aio_req->iocb.ki_complete = ovl_aio_rw_complete;
 		ret = vfs_iocb_iter_write(real.file, &aio_req->iocb, iter);
 		if (ret != -EIOCBQUEUED)
@@ -430,6 +435,10 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 	struct fd real;
 	const struct cred *old_cred;
 	int ret;
+	struct ovl_fs *ofs = OVL_FS(file_inode(file)->i_sb);
+
+	if (ofs->config.nosync)
+		return 0;
 
 	ret = ovl_real_fdget_meta(file, &real, !datasync);
 	if (ret)
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index b429c80879ee..034a8d9897e0 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -17,6 +17,7 @@ struct ovl_config {
 	bool nfs_export;
 	int xino;
 	bool metacopy;
+	bool nosync;
 };
 
 struct ovl_sb {
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 6918b98faeb6..9e93db028dbf 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -863,6 +863,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end,
 	if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
 		return 0;
 
+	if (OVL_FS(dentry->d_sb)->config.nosync)
+		return 0;
+
 	/*
 	 * Need to check if we started out being a lower dir, but got copied up
 	 */
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 91476bc422f9..c28ab39b5c70 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -264,6 +264,8 @@ static int ovl_sync_fs(struct super_block *sb, int wait)
 	if (!ovl_upper_mnt(ofs))
 		return 0;
 
+	if (ofs->config.nosync)
+		return 0;
 	/*
 	 * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC).
 	 * All the super blocks will be iterated, including upper_sb.
@@ -362,6 +364,8 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	if (ofs->config.metacopy != ovl_metacopy_def)
 		seq_printf(m, ",metacopy=%s",
 			   ofs->config.metacopy ? "on" : "off");
+	if (ofs->config.nosync)
+		seq_puts(m, ",nosync");
 	return 0;
 }
 
@@ -376,9 +380,11 @@ static int ovl_remount(struct super_block *sb, int *flags, char *data)
 
 	if (*flags & SB_RDONLY && !sb_rdonly(sb)) {
 		upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
-		down_read(&upper_sb->s_umount);
-		ret = sync_filesystem(upper_sb);
-		up_read(&upper_sb->s_umount);
+		if (!ofs->config.nosync) {
+			down_read(&upper_sb->s_umount);
+			ret = sync_filesystem(upper_sb);
+			up_read(&upper_sb->s_umount);
+		}
 	}
 
 	return ret;
@@ -411,6 +417,7 @@ enum {
 	OPT_XINO_AUTO,
 	OPT_METACOPY_ON,
 	OPT_METACOPY_OFF,
+	OPT_NOSYNC,
 	OPT_ERR,
 };
 
@@ -429,6 +436,7 @@ static const match_table_t ovl_tokens = {
 	{OPT_XINO_AUTO,			"xino=auto"},
 	{OPT_METACOPY_ON,		"metacopy=on"},
 	{OPT_METACOPY_OFF,		"metacopy=off"},
+	{OPT_NOSYNC,			"nosync"},
 	{OPT_ERR,			NULL}
 };
 
@@ -573,6 +581,10 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			metacopy_opt = true;
 			break;
 
+		case OPT_NOSYNC:
+			config->nosync = true;
+			break;
+
 		default:
 			pr_err("unrecognized mount option \"%s\" or missing value\n",
 					p);
@@ -588,6 +600,11 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 		config->workdir = NULL;
 	}
 
+	if (!config->upperdir && config->nosync) {
+		pr_info("option nosync is meaningless in a non-upper mount, ignoring it.\n");
+		config->nosync = false;
+	}
+
 	err = ovl_parse_redirect_mode(config, config->redirect_mode);
 	if (err)
 		return err;
-- 
2.25.4