Date:   Fri, 14 Apr 2017 09:27:20 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        James Simmons <jsimmons@...radead.org>, tahsin@...gle.com,
        nauman@...gle.com, tytso@...gle.com
Subject: Re: [PATCH] ext4: xattr-in-inode support

To summarize the discussion that we had on this week's ext4
teleconference call, while discussing ways in which we might extend
ext4's extended attributes to provide better support for Samba.

Andreas pointed out that we already have an unused field,
e_value_block, in ext4_xattr_entry structure:

struct ext4_xattr_entry {
	__u8	e_name_len;	/* length of name */
	__u8	e_name_index;	/* attribute name index */
	__le16	e_value_offs;	/* offset in disk block of value */
	__le32	e_value_block;	/* disk block attribute is stored on (n/i) */
	__le32	e_value_size;	/* size of attribute value */
	__le32	e_hash;		/* hash value of name and value */
	char	e_name[0];	/* attribute name */
};

It's only a 32-bit field, and it was repurposed in a Lustre-specific
feature, EXT4_FEATURE_INCOMPAT_EA_INODE as e_value_inum (since inodes
are only 32-bit today).  If this feature flag is enabled, then kernels
which understand the feature will treat e_value_block as an inode
number, and if it is non-zero, the value of that extended attribute is
stored in the inode.  This ends up burning a lot of extra inodes for
each extended attribute, which is why there was never much excitement
for this patch going upstream.

However, we could extend this feature (it will almost certainly
require a new INCOMPAT feature flag) such that a particular inode
could be referenced from multiple strut ext4_xattr_entry's (from
multiple inodes or from a single inode), since the inode for the xattr
body already has a ref count, i_links_count.  And given that on a
typical Windows CIFS file system, there will be dozens of unique
acl's, the problem of exhausting inodes for xattrs won't be a issue in
this case.


However, another approach that we discussed on the weekly conference
call was to change e_value_size to be an 16-bit field, and to use the
high 16 bits for flags, where one of the flags bits (say, the MSB)
would mean that e_value_block and e_value_size should be treated as a
48-bit block number, where the block could be stored.

Thinking about this some more, we can use another 4 bits from the high
bits of e_value_size as a 16 bit number n, where if n=0, the block
number is stored in e_Value_block and e_value_size as above, and if n
> 1, that there are additional blocks for the xattr value, which will
be stored in the place where the xattr value would normally be stored
(e.g, in the inline xattr space or in the external xattr block).

So pictorally, it would look like this:

+----------------+----------------+
| 128-byte inode | in-line xattr  |
+----------------+----------------+
                /                  \
               /                    \
              /                      \
  +---------------------------------------------+
  | XE | XE | XE |               | XV | XV | XV |   XE == xattr_entry   XV == xattr value
  +---------------------------------------------+
           /      \             /     \
          /        \           /       \
         /          \         /         \
    +--------------------+  +-------------+
    |   ...  | blk0 |... |  | blk1 | blk2 |
    +--------------------+  +-------------+

(to those using gmail; please view the above in a fixed-width font, or
use "show original")

So in this picture, XE is the ext4_xattr_entry, and in this case, the
high bits of e_value_size indicate e_value_block and the low bits of
e_value_size indicate the location of the first 4k block where the
xattr value is to be stored, and if one were to look at region of
memory indicated by e_value_offs, there would be two 8-byte block
numbers indicating the location of the 2nd and 3rd file system blocks
where the xattr value can be found.

In the external xattr value blocks, at the beginning of the first
block (e.g., at blk0), there will be an ext4_xattr_header, so we can
take advantage of h_refcount field, but with the following changes:

* The low 16 bits of h_blocks will be used for the size of the xattr;
  the high bits of h_blocks must be zero (for now).

* The h_hash field will be a crc32c of the value of the xattr stored
  in the external xattr value block(s).

* The h_checksum field will be calculated so that the crc32c covers
  only the ext4_xattr_header, instead of the entire xattrblock.  e.g.,
  crc32c(fs uuid || id || xattr header), where id is the inode number
  if refcount = 1, and blknum otherwise.

What are the advantages of this approach over the Lustre's
xattr-value-in-inode approach?  First, we don't need to burn inodes
for the xattr value.  This could potentially be an issue for Windows
SID's, since there the number of SID's is roughly equal to number of
users plus the number of groups.  And for a large enterprise with
O(100,000) employees, we could burn a pretty large number of inodes.
The other advantage of this scheme is that h_refcount field is 32
bits, where as the inode's i_links_count field is only 16 bits, and
there could very easily be more than 64k files that might share the
same Windows ACL or Windows SID.  So we would need to figure out some
way of dealing with an extended i_links_count field if we went with
the xattr-value-in-inode approach.


We don't need to make this to be an either-or choice, of course.  We
could integrate the Lustre approach as well as this latter approach
which is more optimized for Windows ACL's.  And I do want to reiterate
that this is just a rough sketch as a design doc.  I'm sure we may
want to make changes to it, but hopefully it will serve as a good
starting point for discussion.

Cheers,

						- Ted


On Thu, Apr 13, 2017 at 01:58:56PM -0600, Andreas Dilger wrote:
> Large xattr support is implemented for EXT4_FEATURE_INCOMPAT_EA_INODE.
> 
> If the size of an xattr value is larger than will fit in a single
> external block, then the xattr value will be saved into the body
> of an external xattr inode.
> 
> The also helps support a larger number of xattr, since only the headers
> will be stored in the in-inode space or the single external block.
> 
> The inode is referenced from the xattr header via "e_value_inum",
> which was formerly "e_value_block", but that field was never used.
> The e_value_size still contains the xattr size so that listing
> xattrs does not need to look up the inode if the data is not accessed.
> 
> struct ext4_xattr_entry {
>  	__u8	e_name_len;	/* length of name */
>  	__u8	e_name_index;	/* attribute name index */
>  	__le16	e_value_offs;	/* offset in disk block of value */
> 	__le32	e_value_inum;	/* inode in which value is stored */
>  	__le32	e_value_size;	/* size of attribute value */
>  	__le32	e_hash;		/* hash value of name and value */
>  	char	e_name[0];	/* attribute name */
> };
> 
> The xattr inode is marked with the EXT4_EA_INODE_FL flag and also
> holds a back-reference to the owning inode in its i_mtime field,
> allowing the ext4/e2fsck to verify the correct inode is accessed.

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