linux-ext4 - Re: [PATCH 1/2] uapi: fix statx attribute value overlap for DAX & MOUNT

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Date:   Tue, 1 Dec 2020 11:44:32 -0600
From:   Eric Sandeen <sandeen@...hat.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     torvalds@...ux-foundation.org,
        Miklos Szeredi <mszeredi@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-man@...r.kernel.org,
        linux-kernel@...r.kernel.org, xfs <linux-xfs@...r.kernel.org>,
        linux-ext4@...r.kernel.org, Xiaoli Feng <xifeng@...hat.com>
Subject: Re: [PATCH 1/2] uapi: fix statx attribute value overlap for DAX &
 MOUNT_ROOT

On 12/1/20 11:32 AM, Darrick J. Wong wrote:
> On Tue, Dec 01, 2020 at 10:57:11AM -0600, Eric Sandeen wrote:
>> STATX_ATTR_MOUNT_ROOT and STATX_ATTR_DAX got merged with the same value,
>> so one of them needs fixing. Move STATX_ATTR_DAX.
>>
>> While we're in here, clarify the value-matching scheme for some of the
>> attributes, and explain why the value for DAX does not match.
>>
>> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
>> ---
>>  include/uapi/linux/stat.h | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
>> index 82cc58fe9368..9ad19eb9bbbf 100644
>> --- a/include/uapi/linux/stat.h
>> +++ b/include/uapi/linux/stat.h
>> @@ -171,9 +171,10 @@ struct statx {
>>   * be of use to ordinary userspace programs such as GUIs or ls rather than
>>   * specialised tools.
>>   *
>> - * Note that the flags marked [I] correspond to generic FS_IOC_FLAGS
>> + * Note that the flags marked [I] correspond to the FS_IOC_SETFLAGS flags
>>   * semantically.  Where possible, the numerical value is picked to correspond
>> - * also.
>> + * also. Note that the DAX attribute indicates that the inode is currently
>> + * DAX-enabled, not simply that the per-inode flag has been set.
> 
> I don't really like using "DAX-enabled" to define "DAX attribute".  How
> about cribbing from the statx manpage?
> 
> "Note that the DAX attribute indicates that the file is in the CPU
> direct access state.  It does not correspond to the per-inode flag that
> some filesystems support."

Sure.  Consistency and specificity is good, I'll change that.

>>   */
>>  #define STATX_ATTR_COMPRESSED		0x00000004 /* [I] File is compressed by the fs */
>>  #define STATX_ATTR_IMMUTABLE		0x00000010 /* [I] File is marked immutable */
>> @@ -183,7 +184,7 @@ struct statx {
>>  #define STATX_ATTR_AUTOMOUNT		0x00001000 /* Dir: Automount trigger */
>>  #define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */
>>  #define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */
>> -#define STATX_ATTR_DAX			0x00002000 /* [I] File is DAX */
>> +#define STATX_ATTR_DAX			0x00400000 /* File is currently DAX-enabled */
> 
> Why not use the next bit in the series (0x200000)?  Did someone already
> claim it in for-next?

Since it didn't match the FS_IOC_SETFLAGS flag, I was trying to pick one that
seemed unlikely to ever gain a corresponding statx flag, and since 0x00400000 is
"reserved for ext4" it seemed like a decent choice.

But 0x200000 corresponds to FS_EA_INODE_FL/EXT4_EA_INODE_FL which is ext4-specific
as well, so sure, I'll change to that.

Thanks,
-Eric

> --D
> 
>>  
>>  
>>  #endif /* _UAPI_LINUX_STAT_H */
>> -- 
>> 2.17.0
>>
>