linux-kernel - Re: Metadata writtenback notification? -- was Re: fscache: Redesigning the on-disk cache

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Date:   Mon, 8 Mar 2021 23:20:18 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     David Howells <dhowells@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, linux-cachefs@...hat.com,
        Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <dchinner@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-afs@...ts.infradead.org,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: Metadata writtenback notification? -- was Re: fscache:
 Redesigning the on-disk cache

On Tue, Mar 09, 2021 at 09:32:47AM +1100, Dave Chinner wrote:
> On Mon, Mar 08, 2021 at 11:28:41AM +0000, David Howells wrote:
> >      Possibly it's sufficient to just clear the excess page space before
> >      writing, but that doesn't necessarily stop a writable mmap from
> >      scribbling on it.
> 
> We can't stop mmap from scribbling in it. All filesystems have this
> problem, so to prevent data leaks we have to zero the post-eof tail
> region on every write of the EOF block, anyway.

That's certainly one approach.  Another would be to zero it during the I/O
completion handler.  It depends whether you can trust the last writer or
not (eg what do we do with an isofs file that happens to contain garbage
after the last byte in the file?)