linux-kernel - Re: [PATCH 3/3] overlayfs: Report writeback errors on upper

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Message-ID: <20201222174637.GK874@casper.infradead.org>
Date:   Tue, 22 Dec 2020 17:46:37 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, jlayton@...nel.org,
        amir73il@...il.com, sargun@...gun.me, miklos@...redi.hu,
        jack@...e.cz, neilb@...e.com, viro@...iv.linux.org.uk, hch@....de
Subject: Re: [PATCH 3/3] overlayfs: Report writeback errors on upper

On Tue, Dec 22, 2020 at 11:29:25AM -0500, Vivek Goyal wrote:
> On Tue, Dec 22, 2020 at 04:20:27PM +0000, Matthew Wilcox wrote:
> > On Mon, Dec 21, 2020 at 02:50:55PM -0500, Vivek Goyal wrote:
> > > +static int ovl_errseq_check_advance(struct super_block *sb, struct file *file)
> > > +{
> > > +	struct ovl_fs *ofs = sb->s_fs_info;
> > > +	struct super_block *upper_sb;
> > > +	int ret;
> > > +
> > > +	if (!ovl_upper_mnt(ofs))
> > > +		return 0;
> > > +
> > > +	upper_sb = ovl_upper_mnt(ofs)->mnt_sb;
> > > +
> > > +	if (!errseq_check(&upper_sb->s_wb_err, file->f_sb_err))
> > > +		return 0;
> > > +
> > > +	/* Something changed, must use slow path */
> > > +	spin_lock(&file->f_lock);
> > > +	ret = errseq_check_and_advance(&upper_sb->s_wb_err, &file->f_sb_err);
> > > +	spin_unlock(&file->f_lock);
> > 
> > Why are you microoptimising syncfs()?  Are there really applications which
> > call syncfs() in a massively parallel manner on the same file descriptor?
> 
> This is atleast theoritical race. I am not aware which application can
> trigger this race. So to me it makes sense to fix the race.
> 
> Jeff Layton also posted a fix for syncfs().
> 
> https://lore.kernel.org/linux-fsdevel/20201219134804.20034-1-jlayton@kernel.org/
> 
> To me it makes sense to fix the race irrespective of the fact if somebody
> hit it or not. People end up copying code in other parts of kernel and
> and they will atleast copy race free code.

Let me try again.  "Why are you trying to avoid taking the spinlock?"