Re: [PATCH v5] overlayfs: Provide a mount option "volatile" to skip sync

[Giuseppe Scrivano <gscrivan@redhat.com>]

Vivek Goyal <vgoyal@redhat.com> writes:

> 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 mount options where they can disable sync on overlay
> mount point.
>
> 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 "volatile" which disables all forms
> of sync. Now it is caller's responsibility to throw away upper if
> system crashes or shuts down and start fresh.
>
> With "volatile", 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).
>
> Changes from v4:
> - Dropped support for sync=fs (Miklos)
> - Renamed "sync=off" to "volatile". (Miklos)
>
> Changes from v3:
> - Used only enums and dropped bit flags (Amir Goldstein)
> - Dropped error when conflicting sync options provided. (Amir Goldstein)
>
> Changes from v2:
> - Added helper functions (Amir Goldstein)
> - Used enums to keep sync state (Amir Goldstein)
>
> 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 |  9 +++++++++
>  fs/overlayfs/copy_up.c                  | 12 ++++++++----
>  fs/overlayfs/file.c                     | 10 +++++++++-
>  fs/overlayfs/ovl_entry.h                |  6 ++++++
>  fs/overlayfs/readdir.c                  |  3 +++
>  fs/overlayfs/super.c                    | 23 ++++++++++++++++++++---
>  6 files changed, 55 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index fcda5d6ba9ac..d5fc5c94560e 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -563,6 +563,15 @@ This verification may cause significant overhead in some cases.
>  Note: the mount options index=off,nfs_export=on are conflicting for a
>  read-write mount 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 "volatile" mount option.
> +
> +"volatile" mount option disables all forms of sync from overlay, including
> +the one done at umount/remount. If system crashes or shuts down, user
> +should throw away upper directory and start fresh.
>  
>  Testsuite
>  ---------
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 5e0cde85bd6b..ffb8334fe94d 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 && ovl_should_sync(ofs))
>  		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 0d940e29d62b..3582c3ae819c 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -331,6 +331,7 @@ 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;
> @@ -346,11 +347,14 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
>  	if (ret)
>  		goto out_unlock;
>  
> +	if (!ovl_should_sync(OVL_FS(inode->i_sb)))
> +		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);
> @@ -370,6 +374,7 @@ 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)
> @@ -433,6 +438,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
>  	const struct cred *old_cred;
>  	int ret;
>  
> +	if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb)))
> +		return 0;
> +
>  	ret = ovl_real_fdget_meta(file, &real, !datasync);
>  	if (ret)
>  		return ret;
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index b429c80879ee..1b5a2094df8e 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 ovl_volatile;
>  };
>  
>  struct ovl_sb {
> @@ -90,6 +91,11 @@ static inline struct ovl_fs *OVL_FS(struct super_block *sb)
>  	return (struct ovl_fs *)sb->s_fs_info;
>  }
>  
> +static inline bool ovl_should_sync(struct ovl_fs *ofs)
> +{
> +	return !ofs->config.ovl_volatile;
> +}
> +
>  /* private information held for every overlayfs dentry */
>  struct ovl_entry {
>  	union {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 6918b98faeb6..2065c10ff0d1 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_should_sync(OVL_FS(dentry->d_sb)))
> +		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 4b38141c2985..701fc4ad822c 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 (!ovl_should_sync(ofs))
> +		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.ovl_volatile)
> +		seq_printf(m, ",volatile");
>  	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 (ovl_should_sync(ofs)) {
> +			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_VOLATILE,
>  	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_VOLATILE,			"volatile"},
>  	{OPT_ERR,			NULL}
>  };
>  
> @@ -573,6 +581,10 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
>  			metacopy_opt = true;
>  			break;
>  
> +		case OPT_VOLATILE:
> +			config->ovl_volatile = true;
> +			break;
> +
>  		default:
>  			pr_err("unrecognized mount option \"%s\" or missing value\n",
>  					p);
> @@ -595,6 +607,11 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
>  		config->index = false;
>  	}
>  
> +	if (!config->upperdir && config->ovl_volatile) {
> +		pr_info("option \"volatile\" is meaningless in a non-upper mount, ignoring it.\n");
> +		config->ovl_volatile = false;
> +	}
> +
>  	err = ovl_parse_redirect_mode(config, config->redirect_mode);
>  	if (err)
>  		return err;

Ping.

Is there anything holding this patch?

Thanks,
Giuseppe