linux-kernel - Re: [PATCH 00/25] Change time_t and clock

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Date:	Thu, 15 May 2014 11:47:03 -0400
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Christoph Hellwig <hch@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ley Foon Tan <lftan@...era.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	LeyFoon Tan <lftan.linux@...il.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

On Wed, 2014-05-14 at 13:00 +0200, Arnd Bergmann wrote:
> On Tuesday 13 May 2014 22:35:08 Geert Uytterhoeven wrote:
> > 
> > On Tue, May 13, 2014 at 9:32 PM, Arnd Bergmann <arnd@...db.de> wrote:
> > > I think we have three categories:
> > 
> > Thanks for the list!
> > 
> > > a) interfaces that uses relative time_t/timespec/timeval:
> > > b) interfaces that don't make sense for times in the past:
> > 
> > > c) interfaces that require absolute times:
> > >    - stat/lstat/fstatat/
> > >    - utime/utimes/futimesat
> > >
> > > These absolutely have to use something better than time_t
> > > both in user space and in the kernel so we can deal with
> > > old files. A lot of file systems need to be fixed as well so
> > > we can actually store the times, regardless of whether we
> > > are running a 32 or 64 bit kernel.
> > 
> > So these are the ones we have to worry about.
> > It looks like they all involve I/O? Apart from the case of using block data
> > from the buffer cache, the 64-bit operations should disappear in the
> > actual I/O noise, right?
> 
> Right. Also there have been proposals for a better 'stat' replacement
> for years, which would solve half of the interface problem for the
> file system interfaces.
> 
> However, we also need to find a solution for category b), I only put
> them into a different category above because we can treat them
> differently in the kernel. For instance, we could use ktime_t for
> the kernel code in category b) and a new struct timespec64 for
> the times in struct inode.
> On the user interface side, using timespec64 would be a reasonable
> choice for both categories, because we already have two implementations
> of all those syscalls in order to handle 32-on-64 compat tasks,
> and we could use the same set of syscall implementations for time64-on-32.

To step back a bit, is a 64 bit time_t actually a good solution on 32
bits?  Paying a 64 bit penalty on every time operation does seem a bit
overkill.  Most eventual uses are either monotonic counting or relative
addition/subtraction.  If we added an additional 32 bit quantity called
epoch, this would increase once every 68 years.  Within the kernel, we
could do intelligent interpolation about what the epoch is, so in 2039
if we get a low 32 bit time_t value, we assume epoch == 1, conversely if
we see a high one, we assume epoch == 0.  We could add epoch on the end
of the syscalls and detect if it's not present and fill in an
interpolated value.  Interfaces which truly want 64 bit time_t would get
it from epoch + 32 bit time_t.

James


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