linux-ext4 - nolazytime remount

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Message-ID: <20150819082144.GB1689@ws.net.home>
Date:	Wed, 19 Aug 2015 10:21:44 +0200
From:	Karel Zak <kzak@...hat.com>
To:	linux-ext4@...r.kernel.org
Cc:	"Theodore Ts'o" <tytso@....edu>
Subject: nolazytime remount

We had issue with "lazytime" remount two mouths ago. This is already
fixed by Ted's patch a2fd66d06. So, now you can enable lazytime by
remount with MS_LAZYTIME flag, but how I can disable lazytime?

enable lazytime (works as expected):

 # findmnt /mnt/test
 TARGET    SOURCE    FSTYPE OPTIONS
 /mnt/test /dev/sdc1 ext4   rw,relatime,stripe=32,data=ordered

 # mount -o remount,lazytime /mnt/test

 # findmnt /mnt/test
 TARGET    SOURCE    FSTYPE OPTIONS
 /mnt/test /dev/sdc1 ext4   rw,relatime,lazytime,stripe=32,data=ordered

disable lazytime by "nolazytime":

 # mount -o remount,nolazytime /mnt/test
 # findmnt /mnt/test
 TARGET    SOURCE    FSTYPE OPTIONS
 /mnt/test /dev/sdc1 ext4   rw,relatime,lazytime,stripe=32,data=ordered
                                        ^^^^^^^^

In this case mount(8) command generates:
  mount("/dev/sdc1", "/mnt/test", 0x562ed2f7e210, MS_REMOUNT|MS_RELATIME, "stripe=32,data=ordered") = 0
syscall.

It seems that ext4_remount() allows to enable the option by MS_LAZYTIME,
but there is no sb->s_flags &= ~MS_LAZYTIME at all. It seems like a
regression, because old string based solution (handle_mount_opt()) contains:

  case Opt_nolazytime:
       sb->s_flags &= ~MS_LAZYTIME;
       return 1;

>From my point of view, a2fd66d06 commit is insufficient, for ext4_remount()
we also need "else" for MS_LAZYTIME:

	if (*flags & MS_LAZYTIME)
		sb->s_flags |= MS_LAZYTIME;
    else
        sb->s_flags &= ~MS_LAZYTIME;

The another possibility is that "lazytime" is possible enable only --
if yes, then handle_mount_opt() should be fixed to not provide
Opt_nolazytime.

Ted?

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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