linux-ext4 - Re: [PATCH] ext4: inherit encryption xattr before other xattrs

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Message-Id: <2917076A-3C7B-4DA4-8A79-CFCEC38007BF@dilger.ca>
Date:   Mon, 27 Feb 2017 15:20:59 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] ext4: inherit encryption xattr before other xattrs

On Feb 27, 2017, at 2:36 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> 
> On Mon, Feb 27, 2017 at 01:28:28PM -0700, Andreas Dilger wrote:
>> On Feb 17, 2017, at 4:33 PM, Eric Biggers <ebiggers3@...il.com> wrote:
>>> 
>>> From: Eric Biggers <ebiggers@...gle.com>
>>> 
>>> When using both encryption and SELinux (or another feature that requires
>>> an xattr per file) on a filesystem with 256-byte inodes, each file's
>>> xattrs usually spill into an external xattr block.  Currently, the
>>> xattrs are inherited in the order ACL, security, then encryption.
>>> Therefore, if spillage occurs, the encryption xattr will always end up
>>> in the external block.  This is not ideal because the encryption xattrs
>>> contain a nonce, so they will always be unique and will prevent the
>>> external xattr blocks from being deduplicated.
>>> 
>>> To improve the situation, change the inheritance order to encryption,
>>> ACL, then security.  This gives the encryption xattr a better chance to
>>> be stored in-inode, allowing the other xattr(s) to be deduplicated.
>>> 
>>> Note that it may be better for userspace to format the filesystem with
>>> 512-byte inodes in this case.  However, it's not the default.
>>> 
>>> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
>> 
>> Reviewed-by: Andreas Dilger <adilger@...ger.ca>
>> 
>> Note that we've been using 512-byte xattrs for Lustre metadata servers
>> for ages.  We may want to consider enabling this by default when the
>> filesystem features are set (e.g. crypto, inline data, etc).
>> 
>> Having a general mechanism to bias xattrs to in-inode or external xattr
>> storage would be nice also, but I don't know any way to do this beyond
>> just having a list of xattr names and then prioritizing the ones that
>> go into the in-inode space.
>> 
> 
> I think it's a good idea to have mke2fs default to 512-byte inodes if
> '-O inline_data' is specified.  But with '-O encrypt' it may be more debatable
> because there is no guarantee as to how many files userspace will actually
> choose to encrypt.  It could be almost the whole filesystem, or just a few
> files, or even nothing at all.
> 
> Regardless, I think adjusting the xattr inheritance order (as this patch does)
> has advantages but no real disadvantages, so it might as well be done too.

Sorry if I wasn't clear.  I have no objections to this patch at all, and would
be happy to see it land.  My comments were just for related improvements that
could be done in different patches.

Cheers, Andreas






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