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Date:   Wed, 8 Jan 2020 08:07:15 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Hugh Dickins <hughd@google.com>, Chris Down <chris@chrisdown.name>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Jeff Layton <jlayton@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, kernel-team@fb.com
Subject: Re: [PATCH v5 2/2] tmpfs: Support 64-bit inums per-sb
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On Tue, Jan 07, 2020 at 12:12:00PM +0200, Amir Goldstein wrote:
> On Tue, Jan 7, 2020 at 10:36 AM Hugh Dickins <hughd@google.com> wrote:
> >
> > On Tue, 7 Jan 2020, Amir Goldstein wrote:
> > > On Tue, Jan 7, 2020 at 2:40 AM Dave Chinner <david@fromorbit.com> wrote:
> > > > On Tue, Jan 07, 2020 at 12:16:43AM +0000, Chris Down wrote:
> > > > > Dave Chinner writes:
> > > > > > It took 15 years for us to be able to essentially deprecate
> > > > > > inode32 (inode64 is the default behaviour), and we were very happy
> > > > > > to get that albatross off our necks.  In reality, almost everything
> > > > > > out there in the world handles 64 bit inodes correctly
> > > > > > including 32 bit machines and 32bit binaries on 64 bit machines.
> > > > > > And, IMNSHO, there no excuse these days for 32 bit binaries that
> > > > > > don't using the *64() syscall variants directly and hence support
> > > > > > 64 bit inodes correctlyi out of the box on all platforms.
> >
> > Interesting take on it.  I'd all along imagined we would have to resort
> > to a mount option for safety, but Dave is right that I was too focused on
> > avoiding tmpfs regressions, without properly realizing that people were
> > very unlikely to have written such tools for tmpfs in particular, but
> > written them for all filesystems, and already encountered and fixed
> > such EOVERFLOWs for other filesystems.
> >
> > Hmm, though how readily does XFS actually reach the high inos on
> > ordinary users' systems?
> >
> 
> Define 'ordinary'
> I my calculations are correct, with default mkfs.xfs any inode allocated
> from logical offset > 2TB on a volume has high ino bits set.
> Besides, a deployment with more than 4G inodes shouldn't be hard to find.

You don't need to allocate 4 billion inodes to get >32bit inodes in
XFS - the inode number is an encoding of the physical location of
the inode in the filesystem. Hence we just have to allocate the
inode at a disk address higher than 2TB into the device and we
overflow 32bits.

e.g. make a sparse fs image file of 10TB, mount it, create 50
subdirs, then start creating zero length files spread across the 50
separate subdirectories. You should see >32bit inode numbers almost
immediately. (i.e. as soon as we allocate inodes in AG 2 or higher)

IOWs, there are *lots* of 64bit inode numbers out there on XFS
filesystems....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com