Received-SPF: pass (google.com: best guess record for domain of linux-ext4-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley
        Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United
        Kingdom.
        Registered in England and Wales under Company Registration No. 3798903
From:   David Howells <dhowells@redhat.com>
In-Reply-To: <20200115144839.GA30301@lst.de>
References: <20200115144839.GA30301@lst.de> <20200115133101.GA28583@lst.de> <4467.1579020509@warthog.procyon.org.uk> <00fc7691-77d5-5947-5493-5c97f262da81@gmx.com> <27181AE2-C63F-4932-A022-8B0563C72539@dilger.ca> <afa71c13-4f99-747a-54ec-579f11f066a0@gmx.com> <26093.1579098922@warthog.procyon.org.uk>
To:     Christoph Hellwig <hch@lst.de>
Cc:     dhowells@redhat.com, Qu Wenruo <quwenruo.btrfs@gmx.com>,
        Andreas Dilger <adilger@dilger.ca>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Problems with determining data presence by examining extents?
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <28754.1579100378.1@warthog.procyon.org.uk>
Content-Transfer-Encoding: quoted-printable
Date:   Wed, 15 Jan 2020 14:59:38 +0000
Message-ID: <28755.1579100378@warthog.procyon.org.uk>
Sender: linux-ext4-owner@vger.kernel.org
Precedence: bulk

Christoph Hellwig <hch@lst.de> wrote:

> > Another thread could be writing to the file, and the raciness matters =
if I
> > want to cache the result of calling SEEK_HOLE - though it might be pos=
sible
> > just to mask it off.
> =

> Well, if you have other threads changing the file (writing, punching hol=
es,
> truncating, etc) you have lost with any interface that isn't an atomic
> give me that data or tell me its a hole.  And even if that if you allow
> threads that aren't part of your fscache implementation to do the
> modifications you have lost.  If on the other hand they are part of
> fscache you should be able to synchronize your threads somehow.

Another thread could be writing to the file at the same time, but not in t=
he
same block.  That's managed by netfs, most likely based on the pages and p=
age
flags attached to the netfs inode being cached in this particular file[*].

What I was more thinking of is that SEEK_HOLE might run past the block of
interest and into a block that's currently being written and see a partial=
ly
written block.

[*] For AFS, this is only true of regular files; dirs and symlinks are cac=
hed
    as monoliths and are there entirely or not at all.

> > However, SEEK_HOLE doesn't help with the issue of the filesystem 'alte=
ring'
> > the content of the file by adding or removing blocks of zeros.
> =

> As does any other method.  If you need that fine grained control you
> need to track the information yourself.

So, basically, I can't.  Okay.  I was hoping it might be possible to add a=
n
ioctl or something to tell filesystems not to do that with particular file=
s.

David