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    SubjectRe: [PATCH] ext4: explain encoding of 34-bit a,c,mtime values
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    On Nov 11, 2013, at 5:30 PM, Theodore Ts'o <tytso@mit.edu> 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. :-)

    Since this change would immediately break files encoded with pre-1970 dates
    on ext4 filesystems, should there be a transition period where both pre-1970
    dates (with extra epoch bits == 0x3 in the current encoding) and post-2378
    (with extra epoch bits == 0x3 in the new encoding) are decoded as being
    pre-1970? That could be conditional until some release in the future (e.g.
    >= Linux 4.20, at least 5 years away) to give folks a chance to run the new
    e2fsck to fix up those files.

    Are there really any ext4 filesystems that have files with valid dates so old?
    I don’t want to break anyone’s data, but if this extra complexity is completely
    pointless then I’m also fine with this minor risk of breakage.

    Cheers, Andreas





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