linux-kernel - Re: [RFC 01/32] fs: introduce new 'struct inode

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Message-ID: <6491264.Vk0CYaEpvf@wuerfel>
Date:	Sat, 31 May 2014 16:53:01 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	joseph@...esourcery.com, john.stultz@...aro.org, hch@...radead.org,
	tglx@...utronix.de, geert@...ux-m68k.org, lftan@...era.com,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 01/32] fs: introduce new 'struct inode_time'

On Saturday 31 May 2014 02:03:38 H. Peter Anvin wrote:
> On 05/30/2014 01:01 PM, Arnd Bergmann wrote:
> > +#ifdef CONFIG_NEW_INODE_TIME
> > +/*
> > + * This is the type we use internally in the kernel to represent
> > + * absolute times in file system metadata.
> > + * This structure must not leak out to user space, and new interfaces
> > + * should be using 64-bit types right away.
> > + */
> > +
> > +/*
> > + * Variant a) using unsigned seconds lets us extend the life span
> > + * for another 69 years beyond 2038.
> > + */
> > +struct inode_time {
> > +     unsigned long   tv_sec;
> > +     long            tv_nsec;
> > +};
> 
> This now differs between 32- and 64-bit systems, and on 32-bit systems
> some timestamps well within the range of representation of current
> systems just became unrepresentable, which is something that I thought
> people were objecting very strongly to.

It really depends on the file system. As you pointed out, I was reading
the ext2/ext3 and xfs code incorrectly, so my assumption when I wrote this
was that they already used the same type, with a 1970-2106 window, rather
than the regular signed Unix epoch.

> > +#elif 0
> > +/*
> > + * This variant can represent the widest range of times, but also
> > + * bloats 'struct inode' a little more.
> > + */
> > +struct inode_time {
> > +     long long       tv_sec __attribute__((packed));
> > +     int             tv_nsec;
> > +};
> 
> Seriously, though, can we really impose constraints stricter than what
> the filesystems themselves do?  It seems we ought to be able to
> represent whatever time the filesystem can represent... (modulo some
> kind of window control as Y2038 or any other break point approaches.)

Just to make sure, do you say we should be using the 'long long/int'
struct, or something else?

	Arnd
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