On 2015-11-24 09:39, Mike Fleetwood wrote:
> On 23 November 2015 at 12:56, Anand Jain <anand.jain@oracle.com> wrote:
>> In the newer kernel, supported kernel features can be known from
>>    /sys/fs/btrfs/features
>> however this interface was introduced only after 3.14, and most the
>> incompatible FS features were introduce before 3.14.
>>
>> This patch proposes to maintain kernel version against the feature list,
>> and so that will be the minimum kernel version needed to use the feature.
>>
>> Further, for features supported later than 3.14 this list can still be
>> updated, so it serves as a repository which can be displayed for easy
>> reference.
>>
>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>> ---
>> v2: Check for condition that what happens when we fail to read kernel
>>      version. Now the code will fail back to use the default as set by
>>      the progs.
>>
>>   utils.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
>>   utils.h |  1 +
>>   2 files changed, 76 insertions(+), 5 deletions(-)
>>
>> diff --git a/utils.c b/utils.c
>> index b754686..24042e5 100644
>> --- a/utils.c
>> +++ b/utils.c
>> @@ -32,10 +32,12 @@
>>   #include <linux/loop.h>
>>   #include <linux/major.h>
>>   #include <linux/kdev_t.h>
>> +#include <linux/version.h>
>>   #include <limits.h>
>>   #include <blkid/blkid.h>
>>   #include <sys/vfs.h>
>>   #include <sys/statfs.h>
>> +#include <sys/utsname.h>
>>   #include <linux/magic.h>
>>
>>   #include "kerncompat.h"
>> @@ -567,21 +569,28 @@ out:
>>          return ret;
>>   }
>>
>> +/*
>> + * min_ker_ver: update with minimum kernel version at which the feature
>> + * was integrated into the mainline. For the transit period, that is
>> + * feature not yet in mainline but in mailing list and for testing,
>> + * please use "0.0" to indicate the same.
>> + */
>>   static const struct btrfs_fs_feature {
>>          const char *name;
>>          u64 flag;
>>          const char *desc;
>> +       const char *min_ker_ver;
>>   } mkfs_features[] = {
>>          { "mixed-bg", BTRFS_FEATURE_INCOMPAT_MIXED_GROUPS,
>> -               "mixed data and metadata block groups" },
>> +               "mixed data and metadata block groups", "2.7.31"},
> I think you mean 2.6.37 here.
> 67377734fd24c3 "Btrfs: add support for mixed data+metadata block groups"
This brings up a rather important question:
Should compat-X.Y mean features that were considered usable in that 
version, or everything that version offered?  I understand wanting 
consistency with the kernel versions, but we shouldn't be creating 
filesystems that we know will break on the specified kernel even if it 
is mountable on it.