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From:   "NeilBrown" <neilb@suse.de>
To:     "Miklos Szeredi" <miklos@szeredi.hu>
Cc:     "Al Viro" <viro@zeniv.linux.org.uk>,
        "Christoph Hellwig" <hch@infradead.org>,
        "Josef Bacik" <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        "Chuck Lever" <chuck.lever@oracle.com>, "Chris Mason" <clm@fb.com>,
        "David Sterba" <dsterba@suse.com>, linux-fsdevel@vger.kernel.org,
        "Linux NFS list" <linux-nfs@vger.kernel.org>,
        "Btrfs BTRFS" <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 01/11] VFS: show correct dev num in mountinfo
In-reply-to: <CAJfpegsR1qvWAKNmdjLfOewUeQy-b6YBK4pcHf7JBExAqqUvvg@mail.gmail.com>
References: <162742539595.32498.13687924366155737575.stgit@noble.brown>,
 <162742546548.32498.10889023150565429936.stgit@noble.brown>,
 <YQNG+ivSssWNmY9O@zeniv-ca.linux.org.uk>,
 <162762290067.21659.4783063641244045179@noble.neil.brown.name>,
 <CAJfpegsR1qvWAKNmdjLfOewUeQy-b6YBK4pcHf7JBExAqqUvvg@mail.gmail.com>
Date:   Fri, 30 Jul 2021 16:13:49 +1000
Message-id: <162762562934.21659.18227858730706293633@noble.neil.brown.name>
Precedence: bulk

On Fri, 30 Jul 2021, Miklos Szeredi wrote:
> On Fri, 30 Jul 2021 at 07:28, NeilBrown <neilb@suse.de> wrote:
> >
> > On Fri, 30 Jul 2021, Al Viro wrote:
> > > On Wed, Jul 28, 2021 at 08:37:45AM +1000, NeilBrown wrote:
> > > > /proc/$PID/mountinfo contains a field for the device number of the
> > > > filesystem at each mount.
> > > >
> > > > This is taken from the superblock ->s_dev field, which is correct for
> > > > every filesystem except btrfs.  A btrfs filesystem can contain multip=
le
> > > > subvols which each have a different device number.  If (a directory
> > > > within) one of these subvols is mounted, the device number reported in
> > > > mountinfo will be different from the device number reported by stat().
> > > >
> > > > This confuses some libraries and tools such as, historically, findmnt.
> > > > Current findmnt seems to cope with the strangeness.
> > > >
> > > > So instead of using ->s_dev, call vfs_getattr_nosec() and use the ->d=
ev
> > > > provided.  As there is no STATX flag to ask for the device number, we
> > > > pass a request mask for zero, and also ask the filesystem to avoid
> > > > syncing with any remote service.
> > >
> > > Hard NAK.  You are putting IO (potentially - network IO, with no upper
> > > limit on the completion time) under namespace_sem.
> >
> > Why would IO be generated? The inode must already be in cache because it
> > is mounted, and STATX_DONT_SYNC is passed.  If a filesystem did IO in
> > those circumstances, it would be broken.
>=20
> STATX_DONT_SYNC is a hint, and while some network fs do honor it, not all d=
o.
>=20

That's ... unfortunate.  Rather seems to spoil the whole point of having
a flag like that.  Maybe it should have been called=20
   "STATX_SYNC_OR_SYNC_NOT_THERE_IS_NO_GUARANTEE"

Thanks.
NeilBrown