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To:     Dave Chinner <david@fromorbit.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     Eric Sandeen <sandeen@redhat.com>, torvalds@linux-foundation.org,
        Miklos Szeredi <mszeredi@redhat.com>,
        Ira Weiny <ira.weiny@intel.com>,
        David Howells <dhowells@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-man@vger.kernel.org,
        linux-kernel@vger.kernel.org, xfs <linux-xfs@vger.kernel.org>,
        linux-ext4@vger.kernel.org, Xiaoli Feng <xifeng@redhat.com>
References: <e388f379-cd11-a5d2-db82-aa1aa518a582@redhat.com>
 <05a0f4fd-7f62-8fbc-378d-886ccd5b3f11@redhat.com>
 <20201201173905.GI143045@magnolia>
 <20201201205243.GK2842436@dread.disaster.area>
From:   Eric Sandeen <sandeen@sandeen.net>
Subject: Re: [PATCH 2/2] statx: move STATX_ATTR_DAX attribute handling to
 filesystems
Message-ID: <9ab51770-1917-fc05-ff57-7677f17b6e44@sandeen.net>
Date:   Tue, 1 Dec 2020 16:03:37 -0600
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On 12/1/20 2:52 PM, Dave Chinner wrote:
> On Tue, Dec 01, 2020 at 09:39:05AM -0800, Darrick J. Wong wrote:
>> On Tue, Dec 01, 2020 at 10:59:36AM -0600, Eric Sandeen wrote:
>>> It's a bit odd to set STATX_ATTR_DAX into the statx attributes in the VFS;
>>> while the VFS can detect the current DAX state, it is the filesystem which
>>> actually sets S_DAX on the inode, and the filesystem is the place that
>>> knows whether DAX is something that the "filesystem actually supports" [1]
>>> so that the statx attributes_mask can be properly set.
>>>
>>> So, move STATX_ATTR_DAX attribute setting to the individual dax-capable
>>> filesystems, and update the attributes_mask there as well.
>>>
>>> [1] 3209f68b3ca4 statx: Include a mask for stx_attributes in struct statx
>>>
>>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>>> ---
>>>  fs/ext2/inode.c   | 6 +++++-
>>>  fs/ext4/inode.c   | 5 ++++-
>>>  fs/stat.c         | 3 ---
>>>  fs/xfs/xfs_iops.c | 5 ++++-
>>>  4 files changed, 13 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
>>> index 11c5c6fe75bb..3550783a6ea0 100644
>>> --- a/fs/ext2/inode.c
>>> +++ b/fs/ext2/inode.c
>>> @@ -1653,11 +1653,15 @@ int ext2_getattr(const struct path *path, struct kstat *stat,
>>>  		stat->attributes |= STATX_ATTR_IMMUTABLE;
>>>  	if (flags & EXT2_NODUMP_FL)
>>>  		stat->attributes |= STATX_ATTR_NODUMP;
>>> +	if (IS_DAX(inode))
>>> +		stat->attributes |= STATX_ATTR_DAX;
>>> +
>>>  	stat->attributes_mask |= (STATX_ATTR_APPEND |
>>>  			STATX_ATTR_COMPRESSED |
>>>  			STATX_ATTR_ENCRYPTED |
>>>  			STATX_ATTR_IMMUTABLE |
>>> -			STATX_ATTR_NODUMP);
>>> +			STATX_ATTR_NODUMP |
>>> +			STATX_ATTR_DAX);
>>>  
>>>  	generic_fillattr(inode, stat);
>>>  	return 0;
>>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>>> index 0d8385aea898..848a0f2b154e 100644
>>> --- a/fs/ext4/inode.c
>>> +++ b/fs/ext4/inode.c
>>> @@ -5550,13 +5550,16 @@ int ext4_getattr(const struct path *path, struct kstat *stat,
>>>  		stat->attributes |= STATX_ATTR_NODUMP;
>>>  	if (flags & EXT4_VERITY_FL)
>>>  		stat->attributes |= STATX_ATTR_VERITY;
>>> +	if (IS_DAX(inode))
>>> +		stat->attributes |= STATX_ATTR_DAX;
>>>  
>>>  	stat->attributes_mask |= (STATX_ATTR_APPEND |
>>>  				  STATX_ATTR_COMPRESSED |
>>>  				  STATX_ATTR_ENCRYPTED |
>>>  				  STATX_ATTR_IMMUTABLE |
>>>  				  STATX_ATTR_NODUMP |
>>> -				  STATX_ATTR_VERITY);
>>> +				  STATX_ATTR_VERITY |
>>> +				  STATX_ATTR_DAX);
>>>  
>>>  	generic_fillattr(inode, stat);
>>>  	return 0;
>>> diff --git a/fs/stat.c b/fs/stat.c
>>> index dacecdda2e79..5bd90949c69b 100644
>>> --- a/fs/stat.c
>>> +++ b/fs/stat.c
>>> @@ -80,9 +80,6 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
>>>  	if (IS_AUTOMOUNT(inode))
>>>  		stat->attributes |= STATX_ATTR_AUTOMOUNT;
>>>  
>>> -	if (IS_DAX(inode))
>>> -		stat->attributes |= STATX_ATTR_DAX;
>>> -
>>>  	if (inode->i_op->getattr)
>>>  		return inode->i_op->getattr(path, stat, request_mask,
>>>  					    query_flags);
>>> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
>>> index 1414ab79eacf..56deda7042fd 100644
>>> --- a/fs/xfs/xfs_iops.c
>>> +++ b/fs/xfs/xfs_iops.c
>>> @@ -575,10 +575,13 @@ xfs_vn_getattr(
>>>  		stat->attributes |= STATX_ATTR_APPEND;
>>>  	if (ip->i_d.di_flags & XFS_DIFLAG_NODUMP)
>>>  		stat->attributes |= STATX_ATTR_NODUMP;
>>> +	if (IS_DAX(inode))
>>> +		stat->attributes |= STATX_ATTR_DAX;
>>>  
>>>  	stat->attributes_mask |= (STATX_ATTR_IMMUTABLE |
>>>  				  STATX_ATTR_APPEND |
>>> -				  STATX_ATTR_NODUMP);
>>> +				  STATX_ATTR_NODUMP |
>>> +				  STATX_ATTR_DAX);
>>
>> TBH I preferred your previous iteration on this, which only set the DAX
>> bit in the attributes_mask if the underlying storage was pmem and the
>> blocksize was correct, etc. since it made it easier to distinguish
>> between a filesystem where you /could/ have DAX (but it wasn't currently
>> enabled) and a filesystem where it just isn't possible.
> 
> I think that's the only thing that makes sense from a userspace
> perspective. THe man page explicitly says that:
> 
>   stx_attributes_mask
> 	A mask indicating which bits in stx_attributes are supported
> 	by the VFS and the filesystem.
> 
> So if DAX can never be turned on for that filesystem instance then,
> by definition, it does not support DAX and the bit should never be
> set.
> 
> e.g. We don't talk about kernels that support reflink - what matters
> to userspace is whether the filesystem instance supports reflink.
> Think of the useless mess that xfs_info would be if it reported
> kernel capabilities instead of filesystem instance capabilities.
> i.e. we don't report that a filesystem supports reflink just because
> the kernel supports it - it reports whether the filesystem instance
> being queried supports reflink. And that also implies the kernel
> supports it, because the kernel has to support it to mount the
> filesystem...
> 
> So, yeah, I think it really does need to be conditional on the
> filesystem instance being queried to be actually useful to users....

So now we're back to "attributes_mask, how does it work?"

The original implementation, as written by the statx interface author, added:

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5d02b922afa3..b9ffa9f4191f 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5413,6 +5413,12 @@ int ext4_getattr(const struct path *path, struct kstat *stat,
        if (flags & EXT4_NODUMP_FL)
                stat->attributes |= STATX_ATTR_NODUMP;
 
+       stat->attributes_mask |= (STATX_ATTR_APPEND |
+                                 STATX_ATTR_COMPRESSED |
+                                 STATX_ATTR_ENCRYPTED |
+                                 STATX_ATTR_IMMUTABLE |
+                                 STATX_ATTR_NODUMP);
+
        generic_fillattr(inode, stat);
        return 0;
 }

setting all those flags /unconditionally/ i.e. STATX_ATTR_ENCRYPTED is always
set in the mask, even if CONFIG_FS_ENCRYPTION=n

And as for compression, that's even better ...

so, um... 

That's why I was keen to just add DAX unconditionally at this point, and if we want
to invent/refine meanings for the mask, we can still try to do that?

-Eric