linux-ext4 - Re: mounting with lazytime doesn't work on ext4

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Date:   Tue, 19 Sep 2017 10:35:06 +0200
From:   Markus Trippelsdorf <markus@...ppelsdorf.de>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org
Subject: Re: mounting with lazytime doesn't work on ext4

On 2017.09.18 at 21:26 +0200, Markus Trippelsdorf wrote:
> I switched back to ext4 yesterday, because my btrfs fs got corrupted.
> However mounting with lazytime doesn't work, neither specifying it in
> /etc/fstab nor a manual remount. It looks like the option is simply
> ignored.
> 
> Strace shows, e.g.:
> 
>  # mount -o lazytime /boot
> mount("/dev/sdc2", "/boot", "ext4", MS_LAZYTIME, NULL) = 0 
> EXT4-fs (sdc2): mounted filesystem with ordered data mode. Opts: (null) 
> /dev/sdc2 on /boot type ext4 (rw,relatime,data=ordered)
> 
>  # mount -o remount,lazytime /var
> mount("/dev/sdb2", "/var", 0x12c4460, MS_REMOUNT|MS_NOATIME|MS_LAZYTIME, NULL) = 0 
> EXT4-fs (sdb2): re-mounted. Opts: (null)
> /dev/sdb2 on /var type ext4 (rw,noatime,data=ordered)
> 
> When I set "sb->s_flags |= MS_LAZYTIME;" unconditionally in
> fs/ext4/super.c:5057 (just deleting the if statement), then lazytime
> gets used when I remount.
> 
> I'm running the latest git tree (4.14.0-rc1).

The following patch seems to fix the issue for remounts:

diff --git a/fs/namespace.c b/fs/namespace.c
index 54059b142d6b..d0b386706c5b 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2322,6 +2322,9 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 	if (err)
 		return err;
 
+	if (mnt_flags & MS_LAZYTIME)
+		sb_flags |= MS_LAZYTIME;
+
 	down_write(&sb->s_umount);
 	if (ms_flags & MS_BIND)
 		err = change_mount_flags(path->mnt, ms_flags);
@@ -2809,6 +2812,8 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 		mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
 	if (flags & SB_RDONLY)
 		mnt_flags |= MNT_READONLY;
+	if (flags & MS_LAZYTIME)
+		mnt_flags |= MS_LAZYTIME;
 
 	/* The default atime for remount is preservation */
 	if ((flags & MS_REMOUNT) &&

-- 
Markus