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

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Date:   Tue, 1 Dec 2020 11:31:28 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Sandeen <sandeen@...hat.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Linux Kernel Mailing List <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 Dec 1, 2020, at 10:44 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> 
> 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.

If you look a few lines up in the context, this is supposed to be using the
same value as the other inode flags:

 * Note that the flags marked [I] correspond to generic FS_IOC_FLAGS
 * semantically.  Where possible, the numerical value is picked to correspond
 * also.

#define FS_DAX_FL                       0x02000000 /* Inode is DAX */
#define EXT4_DAX_FL                     0x02000000 /* Inode is DAX */

(FS_DAX_FL also used by XFS) so this should really be "0x02000000" instead
of some other value.

Cheers, Andreas






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