From: Arnd Bergmann <arnd@arndb.de>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Dave Chinner <david@fromorbit.com>,
hch@infradead.org, linux-mtd@lists.infradead.org,
"H. Peter Anvin" <hpa@zytor.com>,
logfs@logfs.org, linux-afs@lists.infradead.org,
"Joseph S. Myers" <joseph@codesourcery.com>,
linux-arch@vger.kernel.org, linux-cifs@vger.kernel.org,
linux-scsi@vger.kernel.org, ceph-devel@vger.kernel.org,
cluster-devel@redhat.com, coda@cs.cmu.edu, geert@linux-m68k.org,
linux-ext4@vger.kernel.org, codalist@telemann.coda.cs.cmu.edu,
fuse-devel@lists.sourceforge.net, reiserfs-devel@vger.kernel.org,
xfs@oss.sgi.com, john.stultz@linaro.org, tglx@linutronix.de,
linux-nfs@vger.kernel.org, linux-ntfs-dev@lists.sourceforge.net,
samba-technical@lists.samba.org, linux-kernel@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net,
ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org,
lftan@altera.com, linux-btrfs@vger.kernel.org
Subject: Re: [RFC 00/32] making inode time stamps y2038 ready
Date: Wed, 04 Jun 2014 21:24:42 +0200 [thread overview]
Message-ID: <8770583.6XeZxCxOY8@wuerfel> (raw)
In-Reply-To: <alpine.LFD.2.11.1406041308300.17310@knanqh.ubzr>
On Wednesday 04 June 2014 13:30:32 Nicolas Pitre wrote:
> On Wed, 4 Jun 2014, Arnd Bergmann wrote:
>
> > On Tuesday 03 June 2014, Dave Chinner wrote:
> > > Just ot be pedantic, inodes don't need 96 bit timestamps - some
> > > filesystems can *support up to* 96 bit timestamps. If the kernel
> > > only supports 64 bit timestamps and that's all the kernel can
> > > represent, then the upper bits of the 96 bit on-disk inode
> > > timestamps simply remain zero.
> >
> > I meant the reverse: since we have file systems that can store
> > 96-bit timestamps when using 64-bit kernels, we need to extend
> > 32-bit kernels to have the same internal representation so we
> > can actually read those file systems correctly.
> >
> > > If you move the filesystem between kernels with different time
> > > ranges, then the filesystem needs to be able to tell the kernel what
> > > it's supported range is. This is where having the VFS limit the
> > > range of supported timestamps is important: the limit is the
> > > min(kernel range, filesystem range). This allows the filesystems
> > > to be indepenent of the kernel time representation, and the kernel
> > > to be independent of the physical filesystem time encoding....
> >
> > I agree it makes sense to let the kernel know about the limits
> > of the file system it accesses, but for the reverse, we're probably
> > better off just making the kernel representation large enough (i.e.
> > 96 bits) so it can work with any known file system.
>
> Depends... 96 bit handling may get prohibitive on 32-bit archs.
>
> The important point here is for the kernel to be able to represent the
> time _range_ used by any known filesystem, not necessarily the time
> _precision_.
>
> For example, a 64 bit representation can be made of 40 bits for seconds
> spanning 34865 years, and 24 bits for fractional seconds providing
> precision down to 60 nanosecs. That ought to be plenty good on 32 bit
> systems while still being cheap to handle.
I have checked earlier that we don't do any computation on inode
time stamps in common code, we just pass them around, so there is
very little runtime overhead. There is a small bit of space overhead
(12 byte) per inode, but that structure is already on the order of
500 bytes.
For other timekeeping stuff in the kernel, I agree that using some
64-bit representation (nanoseconds, 32/32 unsigned seconds/nanoseconds,
...) has advantages, that's exactly the point I was making earlier
against simply extending the internal time_t/timespec to 64-bit
seconds for everything.
Arnd
next prev parent reply other threads:[~2014-06-04 19:26 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-30 20:01 [RFC 00/32] making inode time stamps y2038 ready Arnd Bergmann
2014-05-30 20:01 ` [RFC 12/32] btrfs: convert to struct inode_time Arnd Bergmann
2014-05-31 14:30 ` [RFC 00/32] making inode time stamps y2038 ready Vyacheslav Dubeyko
2014-06-03 12:21 ` Arnd Bergmann
2014-05-31 14:51 ` Richard Cochran
2014-05-31 15:23 ` Arnd Bergmann
2014-05-31 18:22 ` Richard Cochran
2014-05-31 19:34 ` H. Peter Anvin
2014-06-01 4:46 ` Richard Cochran
2014-06-01 4:44 ` Richard Cochran
2014-06-02 13:52 ` Joseph S. Myers
2014-06-02 19:19 ` Arnd Bergmann
2014-06-02 19:26 ` H. Peter Anvin
2014-06-02 19:55 ` Arnd Bergmann
2014-06-02 21:57 ` H. Peter Anvin
2014-06-03 14:22 ` Arnd Bergmann
2014-06-03 14:33 ` Joseph S. Myers
2014-06-03 14:37 ` Arnd Bergmann
2014-06-03 21:38 ` Dave Chinner
2014-06-04 15:03 ` Arnd Bergmann
2014-06-04 17:30 ` Nicolas Pitre
2014-06-04 19:24 ` Arnd Bergmann [this message]
2014-06-05 0:10 ` H. Peter Anvin
2014-06-10 9:54 ` Arnd Bergmann
2014-06-02 21:02 ` Joseph S. Myers
2014-06-04 15:05 ` Arnd Bergmann
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