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Re: [RFC 00/32] making inode time stamps y2038 ready

To: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
Subject: Re: [RFC 00/32] making inode time stamps y2038 ready
From: Arnd Bergmann <arnd@xxxxxxxx>
Date: Wed, 04 Jun 2014 21:24:42 +0200
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, hch@xxxxxxxxxxxxx, linux-mtd@xxxxxxxxxxxxxxxxxxx, "H. Peter Anvin" <hpa@xxxxxxxxx>, logfs@xxxxxxxxx, linux-afs@xxxxxxxxxxxxxxxxxxx, "Joseph S. Myers" <joseph@xxxxxxxxxxxxxxxx>, linux-arch@xxxxxxxxxxxxxxx, linux-cifs@xxxxxxxxxxxxxxx, linux-scsi@xxxxxxxxxxxxxxx, ceph-devel@xxxxxxxxxxxxxxx, cluster-devel@xxxxxxxxxx, coda@xxxxxxxxxx, geert@xxxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, codalist@xxxxxxxxxxxxxxxxxxxxxxxx, fuse-devel@xxxxxxxxxxxxxxxxxxxxx, reiserfs-devel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, john.stultz@xxxxxxxxxx, tglx@xxxxxxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-ntfs-dev@xxxxxxxxxxxxxxxxxxxxx, samba-technical@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx, ocfs2-devel@xxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, lftan@xxxxxxxxxx, linux-btrfs@xxxxxxxxxxxxxxx
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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

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