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Date:   Fri, 18 Dec 2020 11:28:19 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, amir73il@...il.com,
        sargun@...gun.me, miklos@...redi.hu, willy@...radead.org,
        jack@...e.cz, neilb@...e.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH 3/3] overlayfs: Check writeback errors w.r.t upper in
 ->syncfs()

On Fri, Dec 18, 2020 at 10:02:58AM -0500, Jeff Layton wrote:
> On Fri, Dec 18, 2020 at 09:44:18AM -0500, Vivek Goyal wrote:
> > On Thu, Dec 17, 2020 at 03:08:56PM -0500, Jeffrey Layton wrote:
> > > On Wed, Dec 16, 2020 at 06:31:49PM -0500, Vivek Goyal wrote:
> > > > Check for writeback error on overlay super block w.r.t "struct file"
> > > > passed in ->syncfs().
> > > > 
> > > > As of now real error happens on upper sb. So this patch first propagates
> > > > error from upper sb to overlay sb and then checks error w.r.t struct
> > > > file passed in.
> > > > 
> > > > Jeff, I know you prefer that I should rather file upper file and check
> > > > error directly on on upper sb w.r.t this real upper file.  While I was
> > > > implementing that I thought what if file is on lower (and has not been
> > > > copied up yet). In that case shall we not check writeback errors and
> > > > return back to user space? That does not sound right though because,
> > > > we are not checking for writeback errors on this file. Rather we
> > > > are checking for any error on superblock. Upper might have an error
> > > > and we should report it to user even if file in question is a lower
> > > > file. And that's why I fell back to this approach. But I am open to
> > > > change it if there are issues in this method.
> > > > 
> > > > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > > > ---
> > > >  fs/overlayfs/ovl_entry.h |  2 ++
> > > >  fs/overlayfs/super.c     | 15 ++++++++++++---
> > > >  2 files changed, 14 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> > > > index 1b5a2094df8e..a08fd719ee7b 100644
> > > > --- a/fs/overlayfs/ovl_entry.h
> > > > +++ b/fs/overlayfs/ovl_entry.h
> > > > @@ -79,6 +79,8 @@ struct ovl_fs {
> > > >  	atomic_long_t last_ino;
> > > >  	/* Whiteout dentry cache */
> > > >  	struct dentry *whiteout;
> > > > +	/* Protects multiple sb->s_wb_err update from upper_sb . */
> > > > +	spinlock_t errseq_lock;
> > > >  };
> > > >  
> > > >  static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs)
> > > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> > > > index b4d92e6fa5ce..e7bc4492205e 100644
> > > > --- a/fs/overlayfs/super.c
> > > > +++ b/fs/overlayfs/super.c
> > > > @@ -291,7 +291,7 @@ int ovl_syncfs(struct file *file)
> > > >  	struct super_block *sb = file->f_path.dentry->d_sb;
> > > >  	struct ovl_fs *ofs = sb->s_fs_info;
> > > >  	struct super_block *upper_sb;
> > > > -	int ret;
> > > > +	int ret, ret2;
> > > >  
> > > >  	ret = 0;
> > > >  	down_read(&sb->s_umount);
> > > > @@ -310,10 +310,18 @@ int ovl_syncfs(struct file *file)
> > > >  	ret = sync_filesystem(upper_sb);
> > > >  	up_read(&upper_sb->s_umount);
> > > >  
> > > > +	/* Update overlay sb->s_wb_err */
> > > > +	if (errseq_check(&upper_sb->s_wb_err, sb->s_wb_err)) {
> > > > +		/* Upper sb has errors since last time */
> > > > +		spin_lock(&ofs->errseq_lock);
> > > > +		errseq_check_and_advance(&upper_sb->s_wb_err, &sb->s_wb_err);
> > > > +		spin_unlock(&ofs->errseq_lock);
> > > > +	}
> > > 
> > > So, the problem here is that the resulting value in sb->s_wb_err is
> > > going to end up with the REPORTED flag set (using the naming in my
> > > latest set). So, a later opener of a file on sb->s_wb_err won't see it.
> > > 
> > > For instance, suppose you call sync() on the box and does the above
> > > check and advance. Then, you open the file and call syncfs() and get
> > > back no error because REPORTED flag was set when you opened. That error
> > > will then be lost.
> > 
> > Hi Jeff,
> > 
> > In this patch, I am doing this only in ->syncfs() path and not in
> > ->sync_fs() path. IOW, errseq_check_and_advance() will take place
> > only if there is a valid "struct file" passed in. That means there
> > is a consumer of the error and that means it should be fine to
> > set the sb->s_wb_err as SEEN/REPORTED, right?
> > 
> > If we end up plumbming "struct file" in existing ->sync_fs() routine,
> > then I will call this only if a non NULL struct file has been 
> > passed in. Otherwise skip this step. 
> > 
> > IOW, sync() call will not result in errseq_check_and_advance() instead
> > a syncfs() call will. 
> > 
> 
> It still seems odd and I'm not sure you won't end up with weird corner
> cases due to the flag handling. If you're doing this in the new
> f_op->syncfs, then why bother with sb->s_wb_err at all? You can just do
> this, and avoid the overlayfs sb altogether:
> 
> if (errseq_check(&upper_sb->s_wb_err, file->f_sb_err)) {
> 	/* Upper sb has errors since last time */
> 	spin_lock(&file->f_lock);
> 	errseq_check_and_advance(&upper_sb->s_wb_err, &file->f_sb_err);
> 	spin_unlock(&file->f_lock);
> }
> 
> That's simpler than trying to propagate the error between two
> errseq_t's. You would need to sample the upper_sb->s_wb_err at
> open time in the overlayfs ->open handler though, to make sure
> you're tracking the right one.

IIUC, you are suggesting that when and overlay file is opened (lower or
upper), always install current upper_sb->s_wb_err in f->f_sb_err.
IOW, overide following VFS operations.

f->f_sb_err = file_sample_sb_err(f);

In ovl_open() and ovl_dir_open() with something like.

f->f_sb_err = errseq_sample(upper_sb->s_wb_err); 

And then ->sync_fs() or ->syncfs(), can check for new errors w.r.t upper
sb?

if (errseq_check(&upper_sb->s_wb_err, file->f_sb_err)) {
	/* Upper sb has errors since last time */
	spin_lock(&file->f_lock);
	ret = errseq_check_and_advance(&upper_sb->s_wb_err, &file->f_sb_err);
	spin_unlock(&file->f_lock);
}

I guess I can try this. But if we don't update ovl_sb->s_wb_err, then
question remains that how to avoid errseq_check_and_advance() call
in SYSCALL(sycnfs). That will do more bad things in this case.

This will lead back to either creating new f_op->syncfs() where fs
is responsible for writeback error checks (and not vfs). Or plumb
"struct file" in exisitng ->sync_fs() and let filesystems do
error checks (instead of VFS). This will be somewhat similar to your old
proposal here.

https://lore.kernel.org/linux-fsdevel/20180518123415.28181-1-jlayton@kernel.org/

So advantage of updating ovl_sb->s_wb_err is that it reduces the
churn needed in ->sync_fs() and moving errseq_check_and_advance()
check out of vfs syncfs().

> 
> > > 
> > > >  
> > > > +	ret2 = errseq_check_and_advance(&sb->s_wb_err, &file->f_sb_err);
> > > >  out:
> > > >  	up_read(&sb->s_umount);
> > > > -	return ret;
> > > > +	return ret ? ret : ret2;
> > > >  }
> > > >  
> > > >  /**
> > > > @@ -1903,6 +1911,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> > > >  	if (!cred)
> > > >  		goto out_err;
> > > >  
> > > > +	spin_lock_init(&ofs->errseq_lock);
> > > >  	/* Is there a reason anyone would want not to share whiteouts? */
> > > >  	ofs->share_whiteout = true;
> > > >  
> > > > @@ -1975,7 +1984,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> > > >  
> > > >  		sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth;
> > > >  		sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran;
> > > > -
> > > > +		sb->s_wb_err = errseq_sample(&ovl_upper_mnt(ofs)->mnt_sb->s_wb_err);
> > > 
> > > This will mark the error on the upper_sb as REPORTED, and that's not
> > > really that's the case if you're just using it set s_wb_err in the
> > > overlay. You might want to use errseq_peek in this situation.
> > 
> > For now I am still looking at existing code and not new code. Because
> > I belive that new code does not change existing behavior instead
> > provides additional functionality to allow sampling the error without
> > marking it seen as well as provide helper to not force seeing an
> > unseen error.
> > 
> > So current errseq_sample() does not mark error SEEN. And if it is
> > an unseen error, we will get 0 and be forced to see the error next
> > time.
> > 
> > One small issue with this is that say upper has unseen error. Now
> > we mount overlay and save that value in sb->s_wb_err (unseen). Say
> > a file is opened on upper and error is now seen on upper. But
> > we still have unseen error cached in overlay and if overlay fd is
> > now opened, f->f_sb_err will be 0 and it will be forced to see
> > err on next syncfs().
> > 
> > IOW, despite the fact that overlay fd was opened after upper sb had
> > been marked seen, it still will see error. I think it probably is
> > not a big issue.
> > 
> 
> Good point. I was thinking about the newer code that may mark it
> OBSERVED when you sample at open time.
> 
> Still, I think working with the overlayfs sb->s_wb_err is just adding
> complexity for little benefit.  Assuming that writeback errors can only
> happen on the upper layer, you're better off avoiding it.

If I want to avoid ovl_sb->s_wb_err updation, I will have to move
ret2 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
check in individual filesystems. And it will still not be same. Because
currently after ->sync_fs() call, __sync_blockdev() is called and
then we check for writeback errors. That means, I will have to
move __sync_blockdev() also inside ->sync_fs().

Something like.

fs_sync_fs()
{
	ret = do_fs_specific_sync_stuff();
	ret2 = __sync_blockdev();
	ret3 = errseq_check_and_advance(&sb->s_wb_err, &f.file->f_sb_err);
	if (ret) {
		return ret;
	else
		return ret2 ? ret2 : ret3;
}

Does not look pretty.

Vivek

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