linux-kernel - Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API

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Message-ID: <20210122082322.GB119852@infradead.org>
Date:   Fri, 22 Jan 2021 08:23:22 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Takashi Iwai <tiwai@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        linux-afs@...ts.infradead.org, Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-cachefs@...hat.com, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API

On Thu, Jan 21, 2021 at 06:55:13PM +0000, David Howells wrote:
> > Is it that those "bridging" blocks only show up in certain corner cases
> > that users can arrange to avoid?  Or that it's OK as long as you use
> > certain specific file systems whose behavior goes beyond what's
> > technically required by the bamp or seek interfaces?
> 
> That's a question for the xfs, ext4 and btrfs maintainers, and may vary
> between kernel versions and fsck or filesystem packing utility versions.

For XFS if you do not use reflinks, extent size hints or the RT
subvolume there are no new allocations before i_size that will magically
show up.  But relying on such undocumented assumptions is very
dangerous.