Re: lazytime mount option—no support in Btrfs

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

On 2018-08-22 09:48, David Sterba wrote:
> On Tue, Aug 21, 2018 at 01:01:00PM -0400, Austin S. Hemmelgarn wrote:
>> On 2018-08-21 12:05, David Sterba wrote:
>>> On Tue, Aug 21, 2018 at 10:10:04AM -0400, Austin S. Hemmelgarn wrote:
>>>> On 2018-08-21 09:32, Janos Toth F. wrote:
>>>>>>>> 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'
> 
>> 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).

I also like Duncan's suggestion to expose the default value for the 
atime options as a kconfig option (Chris Murphy emailed me directly 
about essentially the same thing).