Re: [PATCH v2 0/5] Make btrfs-progs really compatible with any kernel version

[Anand Jain <anand.jain@oracle.com>]

David Sterba wrote:
> On Mon, Nov 23, 2015 at 08:56:13PM +0800, Anand Jain wrote:
>> Btrfs-progs is a tool for the btrfs kernel and we hope latest btrfs-progs
>> be compatible w any set of older/newer kernels.
>>
>> So far mkfs.btrfs and btrfs-convert sets the default features, for eg,
>> skinny-metadata even if the running kernel does not supports it, and
>> so the mount fails on the running.
>
> So the default behaviour of mkfs will try to best guess the feature set
> of currently running kernel. I think this is is the most common scenario
> and justifies the change in default behaviours.
>
> For the other cases I'd like to introduce some human-readable shortcuts
> to the --features option. Eg. 'mkfs.btrfs -O compat-3.2' will pick all
> options supported by the unpatched mainline kernel of version 3.2.

  This is a nice idea. I am planning. How about 'as-per-kernel=x.x'
  instead of compat-3.2.

  Also looks like it better to list the feature and version mapping
  as btrfs-progs already knows it at this patchset.


> This
> would be present for all version, regardless if there was a change in the
> options or not.

   Hmm.. I didn't quite get that.

> Similarly for convenience, add 'running' that would pick the options
> from running kernel but will be explicit.
>
> A remaining option should override the 'running' behaviour and pick the
> latest mkfs options. Naming it 'defaults' sounds a bit ambiguous so the
> name is yet to be determined.
>
>> Here in this set of patches will make sure the progs understands the
>> kernel supported features.
>>
>> So in this patch, checks if sysfs tells whether the feature is
>> supported if not, then it will relay on static kernel version which
>> provided that feature (skinny-metadata here in this example), next
>> if for some reason the running kernel does not provide the kernel
>> version, then it will fall back to the original method to enable
>> the feature with a hope that kernel will support it.
>>
>> Also the last patch adds a warning when we fail to read either
>> sysfs features or the running kernel version.
>
> Your patchset is a good start, the additional options I've described can
> be added on top of that.

  Yes.

Thanks, Anand



> We might need to switch the version
> representation from string to KERNEL_VERSION but that's an
> implementation detail.



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