linux-ext4 - Re: [RFC PATCH V2 09/12] fs: Prevent mode change if file is mmap'ed

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Date:   Wed, 15 Jan 2020 12:34:55 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V2 09/12] fs: Prevent mode change if file is mmap'ed

On Tue 14-01-20 09:53:54, Ira Weiny wrote:
> On Mon, Jan 13, 2020 at 05:30:04PM -0800, Darrick J. Wong wrote:
> > > > > +		error = -EBUSY;
> > > > > +		goto out_unlock;
> > > > > +	}
> > > > > +
> > > > >  	error = filemap_write_and_wait(inode->i_mapping);
> > > > >  	if (error)
> > > > >  		goto out_unlock;
> > > > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > > > index 631f11d6246e..6e7dc626b657 100644
> > > > > --- a/include/linux/fs.h
> > > > > +++ b/include/linux/fs.h
> > > > > @@ -740,6 +740,7 @@ struct inode {
> > > > >  #endif
> > > > >  
> > > > >  	void			*i_private; /* fs or device private pointer */
> > > > > +	atomic64_t               i_mapped;
> > > > 
> > > > I would have expected to find this in struct address_space since the
> > > > mapping count is a function of the address space, right?
> > > 
> > > I suppose but the only external call (above) would be passing an inode.  So to
> > > me it seemed better here.
> > 
> > But the number of memory mappings reflects the state of the address
> > space, not the inode.  Or maybe put another way, if I were an mm
> > developer I would not expect to look in struct inode for mm state.
> 
> This is a good point...
> 
> > 
> > static inline bool inode_has_mappings(struct inode *inode)
> > {
> > 	return atomic64_read(&inode->i_mapping->mapcount) > 0;
> > }
> > 
> > OTOH if there exist other mm developers who /do/ find that storing the
> > mmap count in struct inode is more logical, please let me know. :)
> 
> ...  My thinking was that the number of mappings does not matters to the mm
> system...  However, I'm starting to think you are correct...  ;-)
> 
> I've made a note of it and we will see what others think.

Well, more importantly mapping != inode. There can be multiple inodes
pointing to the same mapping (struct address_space) as is the case for
example for block devices. So this counter definitely belongs into struct
address_space.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR