linux-kernel - Re: [PATCH v10 09/14] exfat: add misc operations

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Message-ID: <20200116160051.adn7j7y5oshoe7qt@pali>
Date:   Thu, 16 Jan 2020 17:00:51 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Arnd Bergmann <arnd@...db.de>, Namjae Jeon <linkinjeon@...il.com>,
        Namjae Jeon <namjae.jeon@...sung.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        Valdis Kletnieks <valdis.kletnieks@...edu>,
        sj1557.seo@...sung.com
Subject: Re: [PATCH v10 09/14] exfat: add misc operations

On Thursday 16 January 2020 11:23:07 Christoph Hellwig wrote:
> On Thu, Jan 16, 2020 at 11:19:47AM +0100, Pali Rohár wrote:
> >   However, implementations should only record the value 00h for this
> >   field when:
> > 
> >     1. Local date and time are actually the same as UTC, in which case
> >        the value of the OffsetValid field shall be 1
> > 
> >     2. Local date and time are not known, in which case the value of the
> >        OffsetValid field shall be 1 and implementations shall consider
> >        UTC to be local date and time
> 
> Given time zones in Linux are per session I think our situation is
> somewhat similar to 2.

Seems that 2. is really similar. Ok, storing by default in UTC make
sense, but still I'm thinking if there should be (or not) a mount option
which override default UTC timezone.

> > > Here I would just convert to UTC, which is what we store in the
> > > in-memory struct inode anyway.
> > 
> > Ok. If inode timestamp is always in UTC, we should do same thing also
> > for exFAT.
> 
> > Hm... both UTC and sys_tz have positives and negatives. And I'm not
> > sure which option is better.
> 
> The one big argument for always UTC is simplicity.  Always using UTC
> kills some arcane an unusual (for Linux file systems) code, and given
> how exfat implementations deal with the time zone on reading should
> always interoperate fine with other implementations.

Now I think that using UTC by default is the better option as sys_tz.
Simplicity and "no surprise" (container may use UTC, but kernel has
sys_tz in not in UTC) seems like a good arguments.

-- 
Pali Rohár
pali.rohar@...il.com