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

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Message-ID: <20200114013004.GU8247@magnolia>
Date:   Mon, 13 Jan 2020 17:30:04 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     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 Mon, Jan 13, 2020 at 04:46:10PM -0800, Ira Weiny wrote:
> On Mon, Jan 13, 2020 at 02:22:12PM -0800, Darrick J. Wong wrote:
> > On Fri, Jan 10, 2020 at 11:29:39AM -0800, ira.weiny@...el.com wrote:
> > > From: Ira Weiny <ira.weiny@...el.com>
> > > 
> 
> [snip]
> 
> > >  
> > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > > index bc3654fe3b5d..1ab0906c6c7f 100644
> > > --- a/fs/xfs/xfs_ioctl.c
> > > +++ b/fs/xfs/xfs_ioctl.c
> > > @@ -1200,6 +1200,14 @@ xfs_ioctl_setattr_dax_invalidate(
> > >  		goto out_unlock;
> > >  	}
> > >  
> > > +	/*
> > > +	 * If there is a mapping in place we must remain in our current mode.
> > > +	 */
> > > +	if (atomic64_read(&inode->i_mapped)) {
> > 
> > Urk, should we really be messing around with the address space
> > internals?
> 
> I contemplated a function call instead of checking i_mapped directly?  Is that
> what you mean?

Yeah.  Abstracting the details just enough that filesystems don't have
to know that i_mapped is atomic64 etc.

> 
> > 
> > > +		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.

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. :)

--D

> Ira
> 
> > 
> > --D
> >