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From:   OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
To:     Jeff Layton <jlayton@kernel.org>
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Subject: Re: [PATCH v7 05/13] fat: make fat_update_time get its own timestamp
In-Reply-To: <2cb998ff14ace352a9dd553e82cfa0aa92ec09ce.camel@kernel.org> (Jeff
        Layton's message of "Wed, 09 Aug 2023 12:30:52 -0400")
References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org>
        <20230807-mgctime-v7-5-d1dec143a704@kernel.org>
        <87msz08vc7.fsf@mail.parknet.co.jp>
        <52bead1d6a33fec89944b96e2ec20d1ea8747a9a.camel@kernel.org>
        <878rak8hia.fsf@mail.parknet.co.jp>
        <20230809150041.452w7gucjmvjnvbg@quack3>
        <87v8do6y8q.fsf@mail.parknet.co.jp>
        <2cb998ff14ace352a9dd553e82cfa0aa92ec09ce.camel@kernel.org>
Date:   Thu, 10 Aug 2023 02:44:10 +0900
Message-ID: <87leek6rh1.fsf@mail.parknet.co.jp>
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Jeff Layton <jlayton@kernel.org> writes:

> On Thu, 2023-08-10 at 00:17 +0900, OGAWA Hirofumi wrote:
>> Jan Kara <jack@suse.cz> writes:

[...]

> My mistake re: lazytime vs. relatime, but Jan is correct that this
> shouldn't break anything there.

Actually breaks ("break" means not corrupt fs, means it breaks lazytime
optimization). It is just not always, but it should be always for some
userspaces.

> The logic in the revised generic_update_time is different because FAT is
> is a bit strange. fat_update_time does extra truncation on the timestamp
> that it is handed beyond what timestamp_truncate() does.
> fat_truncate_time is called in many different places too, so I don't
> feel comfortable making big changes to how that works.
>
> In the case of generic_update_time, it calls inode_update_timestamps
> which returns a mask that shows which timestamps got updated. It then
> marks the dirty_flags appropriately for what was actually changed.
>
> generic_update_time is used across many filesystems so we need to ensure
> that it's OK to use even when multigrain timestamps are enabled. Those
> haven't been enabled in FAT though, so I didn't bother, and left it to
> dirtying the inode in the same way it was before, even though it now
> fetches its own timestamps from the clock. Given the way that the mtime
> and ctime are smooshed together in FAT, that seemed reasonable.
>
> Is there a particular case or flag combination you're concerned about
> here?

Yes. Because FAT has strange timestamps that different granularity on
disk . This is why generic time truncation doesn't work for FAT.

Well anyway, my concern is the only following part. In
generic_update_time(), S_[CM]TIME are not the cause of I_DIRTY_SYNC if
lazytime mode.

-	if ((flags & S_VERSION) && inode_maybe_inc_iversion(inode, false))
+	if ((flags & (S_VERSION|S_CTIME|S_MTIME)) && inode_maybe_inc_iversion(inode, false))
		dirty_flags |= I_DIRTY_SYNC;

If reverted this part to check only S_VERSION, I'm fine.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>