Re: lazytime mount option—no support in Btrfs

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2018-08-21 23:57, Duncan wrote:
> Austin S. Hemmelgarn posted on Tue, 21 Aug 2018 13:01:00 -0400 as
> excerpted:
> 
>> Otherwise, the only option for people who want it set is to patch the
>> kernel to get noatime as the default (instead of relatime).  I would
>> look at pushing such a patch upstream myself actually, if it weren't for
>> the fact that I'm fairly certain that it would be immediately NACK'ed by
>> at least Linus, and probably a couple of other people too.
> 
> What about making default-noatime a kconfig option, presumably set to
> default-relatime by default?  That seems to be the way many legacy-
> incompatible changes work.  Then for most it's up to the distro, which in
> fact it is already, only if the distro set noatime-default they'd at
> least be using an upstream option instead of patching it themselves,
> making it upstream code that could be accounted for instead of downstream
> code that... who knows?
That's probably a lot more likely to make it upstream, but it's a bit 
beyond my skills when it comes to stuff like this.
> 
> Meanwhile, I'd be interested in seeing your local patch.  I'm local-
> patching noatime-default here too, but not being a dev, I'm not entirely
> sure I'm doing it "correctly", tho AFAICT it does seem to work.  FWIW,
> here's what I'm doing (posting inline so may be white-space damaged, and
> IIRC I just recently manually updated the line numbers so they don't
> reflect the code at the 2014 date any more, but as I'm not sure of the
> "correctness" it's not intended to be applied in any case):
> 
> --- fs/namespace.c.orig	2014-04-18 23:54:42.167666098 -0700
> +++ fs/namespace.c	2014-04-19 00:19:08.622741946 -0700
> @@ -2823,8 +2823,9 @@ long do_mount(const char *dev_name, cons
>   		goto dput_out;
>   
>   	/* Default to relatime unless overriden */
> -	if (!(flags & MS_NOATIME))
> -		mnt_flags |= MNT_RELATIME;
> +	/* JED: Make that noatime */
> +	if (!(flags & MS_RELATIME))
> +		mnt_flags |= MNT_NOATIME;
>   
>   	/* Separate the per-mountpoint flags */
>   	if (flags & MS_NOSUID)
> @@ -2837,6 +2837,8 @@ long do_mount(const char *dev_name, cons
>   		mnt_flags |= MNT_NOATIME;
>   	if (flags & MS_NODIRATIME)
>   		mnt_flags |= MNT_NODIRATIME;
> +	if (flags & MS_RELATIME)
> +		mnt_flags |= MNT_RELATIME;
>   	if (flags & MS_STRICTATIME)
>   	mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
>   	if (flags & MS_RDONLY)
> 
> Sane, or am I "doing it wrong!"(TM), or perhaps doing it correctly, but
> missing a chunk that should be applied elsewhere?
Mine only has the first part, not the second, which seems to cover 
making sure it's noatime by default.  I never use relatime though, so 
that may be broken with my patch because of me not having the second part.
> 
> 
> Meanwhile, since broken rootflags requiring an initr* came up let me take
> the opportunity to ask once again, does btrfs-raid1 root still require an
> initr*?  It'd be /so/ nice to be able to supply the appropriate
> rootflags=device=...,device=... and actually have it work so I didn't
> need the initr* any longer!
Last I knew, specifying appropriate `device=` options in rootflags works 
correctly without an initrd.