linux-ext4 - Re: [PATCH] ext4: explain encoding of 34-bit a,c,mtime values

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Message-ID: <1384310211.8994.161.camel@chiang>
Date:	Tue, 12 Nov 2013 21:36:51 -0500
From:	David Turner <novalis@...alis.org>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Theodore Ts'o <tytso@....edu>, Mark Harris <mhlk@....us>,
	Andreas Dilger <adilger@...ger.ca>, Jan Kara <jack@...e.cz>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ext4: explain encoding of 34-bit a,c,mtime values

On Tue, 2013-11-12 at 15:03 -0800, Darrick J. Wong wrote:
> On Mon, Nov 11, 2013 at 07:30:18PM -0500, Theodore Ts'o wrote:
> > On Sun, Nov 10, 2013 at 02:56:54AM -0500, David Turner wrote:
> > > b. Use Andreas's encoding, which is incompatible with pre-1970 files
> > > written on 64-bit systems.
> > >
> > > I don't care about currently-existing post-2038 files, because I believe
> > > that nobody has a valid reason to have such files.  However, I do
> > > believe that pre-1970 files are probably important to someone.
> > > 
> > > Despite this, I prefer option (b), because I think the simplicity is
> > > valuable, and because I hate to give up date ranges (even ones that I
> > > think we'll "never" need). Option (b) is not actually lossy, because we
> > > could correct pre-1970 files with e2fsck; under Andreas's encoding,
> > > their dates would be in the far future (and thus cannot be legitimate).
> > > 
> > > Would a patch that does (b) be accepted?  I would accompany it with a
> > > patch to e2fsck (which I assume would also go to the ext4 developers
> > > mailing list?).
> > 
> > I agree, I think this is the best way to go.  I'm going to drop your
> > earlier patch, and wait for an updated patch from you.  It may miss
> > this merge window, but as Andreas has pointed out, we still have a few
> > years to get this right.  :-)
> 
> Just to be clear... we're going with Andreas' fix, wherein
> 
> time->tv_sec |= (__u64)(le32_to_cpu(extra) & EXT4_EPOCH_MASK) << 32;
> 
> becomes:
> 
> time->tv_sec += (__u64)(le32_to_cpu(extra) & EXT4_EPOCH_MASK) << 32;
> 
> "or" becomes "plus"?  So I can update fuse2fs.

Yes, but with a kernel-version-dependent change, which is something like
this:
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4,20,0)
		time->tv_sec += (u64)(le32_to_cpu(extra) & EXT4_EPOCH_MASK) << 32;
#else
		u64 extra_bits = le32_to_cpu(extra) & EXT4_EPOCH_MASK;
		if (extra_bits == 3)
			extra_bits = 0;
		time->tv_sec += extra_bits << 32;
#endif

> Also, can someone proofread [1] and make sure it's correct?

It's not quite right.  I've requested an account so that I can correct
it.

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