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From: Vivek Goyal <vgoyal@redhat.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	Giuseppe Scrivano <gscrivan@redhat.com>,
	Daniel J Walsh <dwalsh@redhat.com>,
	Steven Whitehouse <swhiteho@redhat.com>,
	pmatilai@redhat.com, sandeen@redhat.com
Subject: Re: [PATCH v4] overlayfs: Provide mount options sync=off/fs to skip sync
Date: Mon, 20 Jul 2020 12:16:18 -0400
Message-ID: <20200720161618.GD502563@redhat.com> (raw)
In-Reply-To: <CAJfpegtBjv60ZYJYSgQfU9EFx+eMbjqzcZ1HFV8P2nL64x5D2A@mail.gmail.com>

On Mon, Jul 20, 2020 at 02:57:54PM +0200, Miklos Szeredi wrote:
> On Mon, Jul 6, 2020 at 6:12 PM Vivek Goyal <vgoyal@redhat.com> 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.

Hi Miklos,

For building images containers folks need to sync upper layer. Their
current plan is to use "syncfs upper/" because it is same as if overlay
was mounted with sync=fs. But this syncs whole upper filesystem and
not just upper of a particular overlayfs instance

So idea was to provide sync=fs from the beginning and ask container
folks to use this. So that in future if we can optimize sync=fs to
sync selctive inodes, then container runtime will automatically
benefit from it without any changes. It also reduces the chances
of error on container runtime which fail to sync upper.  Hence idea
of sync=fs sounded appleaing to me. 

Havid said that, I am open to dropping sync=fs for now, if you don't
see the value at this point of time.

> 
> 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.

I am fine with the name "volatile/temporary" for sync=off.  

Amir, WDYT?

Vivek

> 
> 
> 
> 
> 
> >
> > 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@redhat.com>
> > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> >  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
> >
> 


  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-06 16:12 Vivek Goyal
2020-07-06 17:16 ` Amir Goldstein
2020-07-20 12:57 ` Miklos Szeredi
2020-07-20 16:16   ` Vivek Goyal [this message]
2020-07-20 17:47     ` Amir Goldstein
2020-07-21 13:15     ` Miklos Szeredi
2020-07-21 13:44       ` Amir Goldstein
2020-07-21 13:51       ` Vivek Goyal

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