Message-ID: <CAOQ4uxgC7OJ+TF0Gw-ZdSTpq1goGgfmgWNwsx_hqOwiiFB-UHg@mail.gmail.com>
Date: Sat, 20 Jul 2024 10:29:47 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Lv Fei（吕飞） <feilv@...micro.com>
Cc: "miklos@...redi.hu" <miklos@...redi.hu>, 
	"linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	Xu Lianghu（徐良虎） <lianghuxu@...micro.com>
Subject: Re: [PATCH] ovl: fsync after metadata copy-up via mount option "upsync=strict"

On Fri, Jul 19, 2024 at 12:55 PM Lv Fei（吕飞） <feilv@...micro.com> wrote:
>
>
> >
> > -----邮件原件-----
> > 发件人: Amir Goldstein [mailto:amir73il@...il.com]
> > 发送时间: 2024年7月19日 15:24
> > 收件人: Lv Fei（吕飞） <feilv@...micro.com>
> > 抄送: miklos@...redi.hu; linux-unionfs@...r.kernel.org; linux-kernel@...r.kernel.org; Xu Lianghu（徐良虎） > <lianghuxu@...micro.com>
> > 主题: Re: [PATCH] ovl: fsync after metadata copy-up via mount option "upsync=strict"
> >
> > On Thu, Jul 18, 2024 at 6:43 AM Fei Lv <feilv@...micro.com> wrote:
> > >
> > > If a directory only exist in low layer, create a new file in it
> > > trigger directory copy-up. Permission lost of the new directory in
> > > upper layer was observed during power-cut stress test.
> >
> > You should specify that this outcome happens on very specific upper fs (i.e. ubifs) which does not enforce ordering on storing of metadata changes.
>
> OK.
>
> >
> > >
> > > Fix by adding new mount opion "upsync=strict", make sure data/metadata
> > > of copied up directory written to disk before renaming from tmp to
> > > final destination.
> > >
> > > Signed-off-by: Fei Lv <feilv@...micro.com>
> > > ---
> > > OPT_sync changed to OPT_upsync since mount option "sync" already used.
> >
> > I see. I don't like the name "upsync" so much, it has other meanings how about using the option "fsync"?
>
> OK.
>
> >
> > Here is a suggested documentation (which should be accompanied to any patch)
>
> OK.
>
> >
> > diff --git a/Documentation/filesystems/overlayfs.rst
> > b/Documentation/filesystems/overlayfs.rst
> > index 165514401441..f8183ddf8c4d 100644
> > --- a/Documentation/filesystems/overlayfs.rst
> > +++ b/Documentation/filesystems/overlayfs.rst
> > @@ -742,6 +742,42 @@ controlled by the "uuid" mount option, which supports these values:
> >      mounted with "uuid=on".
> >
> >
> > +Durability and copy up
> > +----------------------
> > +
> > +The fsync(2) and fdatasync(2) system calls ensure that the metadata and
> > +data of a file, respectively, are safely written to the backing
> > +storage, which is expected to guarantee the existence of the information post system crash.
> > +
> > +Without the fdatasync(2) call, there is no guarantee that the observed
> > +data after a system crash will be either the old or the new data, but
> > +in practice, the observed data after crash is often the old or new data or a mix of both.
> > +
> > +When overlayfs file is modified for the first time, copy up will create
> > +a copy of the lower file and its parent directories in the upper layer.
> > +In case of a system crash, if fdatasync(2) was not called after the
> > +modification, the upper file could end up with no data at all (i.e.
> > +zeros), which would be an unusual outcome.  To avoid this experience,
> > +overlayfs calls fsync(2) on the upper file before completing the copy up with rename(2) to make the copy > up "atomic".
> > +
> > +Depending on the backing filesystem (e.g. ubifs), fsync(2) before
> > +rename(2) may not be enough to provide the "atomic" copy up behavior
> > +and fsync(2) on the copied up parent directories is required as well.
> > +
> > +Overlayfs can be tuned to prefer performance or durability when storing
> > +to the underlying upper layer.  This is controlled by the "fsync" mount
> > +option, which supports these values:
> > +
> > +- "ordered": (default)
> > +    Call fsync(2) on upper file before completion of copy up.
> > +- "strict":
> > +    Call fsync(2) on upper file and directories before completion of copy up.
> > +- "volatile": [*]
> > +    Prefer performance over durability (see `Volatile mount`_)
> > +
> > +[*] The mount option "volatile" is an alias to "fsync=volatile".
> > +
> > +
> >  Volatile mount
> >  --------------
> >
> > >
> > >  fs/overlayfs/copy_up.c   | 21 +++++++++++++++++++++
> > >  fs/overlayfs/ovl_entry.h | 20 ++++++++++++++++++--
> > >  fs/overlayfs/params.c    | 33 +++++++++++++++++++++++++++++----
> > >  fs/overlayfs/super.c     |  2 +-
> > >  4 files changed, 69 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c index
> > > a5ef2005a2cc..b6f021ad7a43 100644
> > > --- a/fs/overlayfs/copy_up.c
> > > +++ b/fs/overlayfs/copy_up.c
> > > @@ -243,6 +243,21 @@ static int ovl_verify_area(loff_t pos, loff_t pos2, loff_t len, loff_t totlen)
> > >         return 0;
> > >  }
> > >
> > > +static int ovl_copy_up_sync(struct path *path) {
> > > +       struct file *new_file;
> > > +       int err;
> > > +
> > > +       new_file = ovl_path_open(path, O_LARGEFILE | O_WRONLY);
> >
> > I don't think any of those O_ flags are needed for fsync.
> > Can a directory be opened O_WRONLY???
>
> This function may be called with file or directory, shall I need to use different
> flags? Such as below:
>
> static int ovl_copy_up_sync(struct path *path, bool is_dir)
> {
>         struct file *new_file;
>         int flags;
>         int err;
>
>         flags = is_dir ? (O_RDONLY | O_DIRECTORY) : (O_LARGEFILE | O_WRONLY);
>         new_file = ovl_path_open(path, flags);
>         if (IS_ERR(new_file))
>                 return PTR_ERR(new_file);
>
>         err = vfs_fsync(new_file, 0);
 >         fput(new_file);
>
>         return err;
> }
>

You do not need O_WRONLY nor O_LARGEFILE for fsync of a regular file
just use O_RDONLY unconditionally.

> >
> > > +       if (IS_ERR(new_file))
> > > +               return PTR_ERR(new_file);
> > > +
> > > +       err = vfs_fsync(new_file, 0);
> > > +       fput(new_file);
> > > +
> > > +       return err;
> > > +}
> > > +
> > >  static int ovl_copy_up_file(struct ovl_fs *ofs, struct dentry *dentry,
> > >                             struct file *new_file, loff_t len)  { @@
> > > -701,6 +716,9 @@ static int ovl_copy_up_metadata(struct ovl_copy_up_ctx *c, struct dentry *temp)
> > >                 err = ovl_set_attr(ofs, temp, &c->stat);
> > >         inode_unlock(temp->d_inode);
> > >
> > > +       if (!err && ovl_should_sync_strict(ofs))
> > > +               err = ovl_copy_up_sync(&upperpath);
> > > +
> > >         return err;
> > >  }
> > >
> > > @@ -1104,6 +1122,9 @@ static int ovl_copy_up_meta_inode_data(struct ovl_copy_up_ctx *c)
> > >         ovl_clear_flag(OVL_HAS_DIGEST, d_inode(c->dentry));
> > >         ovl_clear_flag(OVL_VERIFIED_DIGEST, d_inode(c->dentry));
> > >         ovl_set_upperdata(d_inode(c->dentry));
> > > +
> > > +       if (!err && ovl_should_sync_strict(ofs))
> > > +               err = ovl_copy_up_sync(&upperpath);
> >
> > fsync was probably already called in ovl_copy_up_file() making this call redundant and fsync of the removal > of metacopy xattr does not add any safety.
>
> My original idea was that ovl_should_sync and ovl_should_sync_strict should not be enabled at the same time.

You have it wrong.

The ovl_should_sync() helper does not mean sync_mode==ordered,
it means sync_mode!=volatile
It literally means "should overlayfs respect fsync"
and it is used in several places in the code.

So ovl_should_sync_strict() always implies ovl_should_sync().

> The reasons are as follows:
> If bothe ovl_should_sync and ovl_should_sync_strict return ture for "fsync=strict",
> and power cut between ovl_copy_up_file and ovl_copy_up_metadata for file copy-up,
> seems this file may also lost permission?

fsync of file in ovl_copy_up_file() the file is either an O_TMPFILE
or in the workdir. no risk involved even with ubifs.

>
> >
> > >  out_free:
> > >         kfree(capability);
> > >  out:
> > > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h index
> > > cb449ab310a7..4592e6f7dcf7 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_DATA,
> > > +       OVL_SYNC_STRICT,
> > > +       OVL_SYNC_OFF,
> > > +};
> > > +
> > >  struct ovl_config {
> > >         char *upperdir;
> > >         char *workdir;
> > > @@ -18,7 +24,7 @@ struct ovl_config {
> > >         int xino;
> > >         bool metacopy;
> > >         bool userxattr;
> > > -       bool ovl_volatile;
> > > +       int sync_mode;
> > >  };
> > >
> > >  struct ovl_sb {
> > > @@ -120,7 +126,17 @@ static inline struct ovl_fs *OVL_FS(struct
> > > super_block *sb)
> > >
> > >  static inline bool ovl_should_sync(struct ovl_fs *ofs)  {
> > > -       return !ofs->config.ovl_volatile;
> > > +       return ofs->config.sync_mode == OVL_SYNC_DATA;
> >
> >     return ofs->config.sync_mode != OVL_SYNC_OFF; or
> >     return ofs->config.sync_mode != OVL_FSYNC_VOLATILE;
>
> There are risks if ovl_should_sync and ovl_should_sync_strict enabled at the same time.
> The reasons are above.
>

No. see above.

Let me know if I misunderstood your concern.

Thanks,
Amir.

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