On 2018-08-22 11:01, David Sterba wrote:

On Wed, Aug 22, 2018 at 09:56:59AM -0400, Austin S. Hemmelgarn wrote:

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On 2018-08-22 09:48, David Sterba wrote:

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On Tue, Aug 21, 2018 at 01:01:00PM -0400, Austin S. Hemmelgarn wrote:

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On 2018-08-21 12:05, David Sterba wrote:

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On Tue, Aug 21, 2018 at 10:10:04AM -0400, Austin S. Hemmelgarn wrote:

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On 2018-08-21 09:32, Janos Toth F. wrote:

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so pretty much everyone who wants to avoid the overhead from them can just
use the `noatime` mount option.

It would be great if someone finally fixed this old bug then:
https://bugzilla.kernel.org/show_bug.cgi?id=61601
Until then, it seems practically impossible to use both noatime (this
can't be added as rootflag in the command line and won't apply if the
kernel already mounted the root as RW) and space-cache-v2 (has to be
added as a rootflag along with RW to take effect) for the root
filesystem (at least without an init*fs, which I never use, so can't
tell).

Last I knew, it was fixed.  Of course, it's been quite a while since I
last tried this, as I run locally patched kernels that have `noatime` as
the default instead of `relatime`.

I'm using VMs without initrd, tested the rootflags=noatime and it still
fails, the same way as in the bugreport.

As the 'noatime' mount option is part of the mount(2) API (passed as a
bit via mountflags), the remaining option in the filesystem is to
whitelist the generic options and ignore them. But this brings some
layering violation question.

On the other hand, this would be come confusing as the user expectation
is to see the effects of 'noatime'.

Ideally there would be a way to get this to actually work properly.  I
think ext4 at least doesn't panic, though I'm not sure if it actually
works correctly.

No, ext4 also refuses to mount, the panic happens in VFS that tries
either the rootfstype= or all available filesystems.

[    3.763602] EXT4-fs (sda): Unrecognized mount option "noatime" or missing value

[    3.761315] BTRFS info (device sda): unrecognized mount option 'noatime'

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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.

An acceptable solution could be to parse the rootflags and translate
them to the MNT_* values, ie. what the commandline tool mount does
before it calls the mount syscall.

That would be helpful, but at that point you might as well update the
CLI mount tool to just pass all the named options to the kernel and have
it do the parsing (I mean, keep the old interface too obviously, but
provide a new one and use that preferentially).

The initial mount is not done by the mount tool but internally by
kernel init sequence (files in init/):

mount_block_root
   do_mount_root
     ksys_mount

The mount options (as a string) is passed unchanged via variable
root_mount_data (== rootflags). So before this step, the options would
have to be filtered and all known generic options turned into bit flags.

What I'm saying is that if there's going to be parsing for it in the 
kernel anyway, why not expose that interface to userspace too so that 
the regular `mount` tool can take advantage of it as well.