On Mon, Jul 6, 2020 at 6:12 PM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> 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 two mount options "sync=off" and "sync=fs".
> First option disables all forms of sync. Now it is caller's responsibility
> to throw away upper if system crashes or shuts down and start fresh.
>
> Option "sync=fs" disables all forms of sync except syncfs/umount/remount.
> This is basically useful for image build where we want to persist upper
> layer only after operation is complete and upper will be renamed and
> reused as lower for next layer build.
>
> With sync=off, 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.
>
> With sync=fs, I am seeing roughly 12% speed up.
I think we do need some sort of protection against careless multiple
mounting of such filesystem. Implementing that shouldn't be a big
issue: verify that file doesn't exist, regardless of sync mode, create
file in no-sync mode (and remove it on shutdown in sync=fs mode, but
see below).
I agree about the usefulness of an fs-sync (or sync on shutdown) mode.
However container folks said they don't need that so for now I think
you should drop the sync=fs mode.
Naming: I'm not at all convinced by any name having "sync" in it. I
think "sync=no" is about the implementation, not the functionality,
and so it's confusing. The functionality is better described by
"volatile" or "temporary". But I can live with sync=... if voted
down.
Thanks,
Miklos
>
> 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 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@xxxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---
> Documentation/filesystems/overlayfs.rst | 16 +++++++++++
> fs/overlayfs/copy_up.c | 12 ++++++---
> fs/overlayfs/file.c | 10 ++++++-
> fs/overlayfs/ovl_entry.h | 17 ++++++++++++
> fs/overlayfs/readdir.c | 3 +++
> fs/overlayfs/super.c | 35 ++++++++++++++++++++++---
> 6 files changed, 85 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 660dbaf0b9b8..4e55ac4433ec 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -563,6 +563,22 @@ 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 "sync=off" and "sync=fs" mount option.
> +
> +"sync=off" 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.
> +
> +"sync=fs" option disables all forms of sync except full filesystem
> +sync which is done at syncfs/remount/mount time. This is useful for
> +use cases like container image build which want upper to persist
> +only if operation has finished. If system crashes before image
> +layer formation is complete, tools should discard upper and start
> +fresh.
>
> Testsuite
> ---------
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 79dd052c7dbf..3a5ae9c2f86e 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_fsync(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 01820e654a21..c92af3856dbf 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_should_fsync(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);
> @@ -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)
> @@ -431,6 +436,9 @@ 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_fsync(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..e6d21eff5620 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -5,6 +5,12 @@
> * Copyright (C) 2016 Red Hat, Inc.
> */
>
> +enum ovl_sync_type {
> + OVL_SYNC_ON,
> + OVL_SYNC_OFF,
> + OVL_SYNC_FS,
> +};
> +
> struct ovl_config {
> char *lowerdir;
> char *upperdir;
> @@ -17,6 +23,7 @@ struct ovl_config {
> bool nfs_export;
> int xino;
> bool metacopy;
> + enum ovl_sync_type sync;
> };
>
> struct ovl_sb {
> @@ -90,6 +97,16 @@ static inline struct ovl_fs *OVL_FS(struct super_block *sb)
> return (struct ovl_fs *)sb->s_fs_info;
> }
>
> +static inline bool ovl_should_fsync(struct ovl_fs *ofs)
> +{
> + return ofs->config.sync == OVL_SYNC_ON;
> +}
> +
> +static inline bool ovl_should_syncfs(struct ovl_fs *ofs)
> +{
> + return ofs->config.sync != OVL_SYNC_OFF;
> +}
> +
> /* 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..80f772faad5c 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_fsync(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 91476bc422f9..04f6108fdc69 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_syncfs(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.
> @@ -327,6 +329,12 @@ static const char * const ovl_xino_str[] = {
> "on",
> };
>
> +static const char * const ovl_sync_str[] = {
> + "on",
> + "off",
> + "fs",
> +};
> +
> static inline int ovl_xino_def(void)
> {
> return ovl_xino_auto_def ? OVL_XINO_AUTO : OVL_XINO_OFF;
> @@ -362,6 +370,8 @@ 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.sync != OVL_SYNC_ON)
> + seq_printf(m, ",sync=%s", ovl_sync_str[ofs->config.sync]);
> return 0;
> }
>
> @@ -376,9 +386,11 @@ 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_syncfs(ofs)) {
> + down_read(&upper_sb->s_umount);
> + ret = sync_filesystem(upper_sb);
> + up_read(&upper_sb->s_umount);
> + }
> }
>
> return ret;
> @@ -411,6 +423,8 @@ enum {
> OPT_XINO_AUTO,
> OPT_METACOPY_ON,
> OPT_METACOPY_OFF,
> + OPT_SYNC_OFF,
> + OPT_SYNC_FS,
> OPT_ERR,
> };
>
> @@ -429,6 +443,8 @@ static const match_table_t ovl_tokens = {
> {OPT_XINO_AUTO, "xino=auto"},
> {OPT_METACOPY_ON, "metacopy=on"},
> {OPT_METACOPY_OFF, "metacopy=off"},
> + {OPT_SYNC_OFF, "sync=off"},
> + {OPT_SYNC_FS, "sync=fs"},
> {OPT_ERR, NULL}
> };
>
> @@ -573,6 +589,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
> metacopy_opt = true;
> break;
>
> + case OPT_SYNC_OFF:
> + config->sync = OVL_SYNC_OFF;
> + break;
> +
> + case OPT_SYNC_FS:
> + config->sync = OVL_SYNC_FS;
> + break;
> +
> default:
> pr_err("unrecognized mount option \"%s\" or missing value\n",
> p);
> @@ -588,6 +612,11 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
> config->workdir = NULL;
> }
>
> + if (!config->upperdir && config->sync) {
> + pr_info("option sync=off/fs is meaningless in a non-upper mount, ignoring it.\n");
> + config->sync = 0;
> + }
> +
> err = ovl_parse_redirect_mode(config, config->redirect_mode);
> if (err)
> return err;
> --
> 2.25.4
>
