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7d3e06a8da
We prevent DAX from being used on inodes which are using ext4's built in encryption via a check in ext4_set_inode_flags(). We do have what appears to be an unsafe transition of S_DAX in ext4_set_context(), though, where S_DAX can get disabled without us doing a proper writeback + invalidate. There are also issues with mm-level races when changing the value of S_DAX, as well as issues with the VM_MIXEDMAP flag: https://www.spinics.net/lists/linux-xfs/msg09859.html I actually think we are safe in this case because of the following: 1) You can't encrypt an existing file. Encryption can only be set on an empty directory, with new inodes in that directory being created with encryption turned on, so I don't think it's possible to turn encryption on for a file that has open DAX mmaps or outstanding I/Os. 2) There is no way to turn encryption off on a given file. Once an inode is encrypted, it stays encrypted for the life of that inode, so we don't have to worry about the case where we turn encryption off and S_DAX suddenly turns on. 3) The only way we end up in ext4_set_context() to turn on encryption is when we are creating a new file in the encrypted directory. This happens as part of ext4_create() before the inode has been allowed to do any I/O. Here's the call tree: ext4_create() __ext4_new_inode() ext4_set_inode_flags() // sets S_DAX fscrypt_inherit_context() fscrypt_get_encryption_info(); ext4_set_context() // sets EXT4_INODE_ENCRYPT, clears S_DAX So, I actually think it's safe to transition S_DAX in ext4_set_context() without any locking, writebacks or invalidations. I've added a WARN_ON_ONCE() sanity check to make sure that we are notified if we ever encounter a case where we are encrypting an inode that already has data, in which case we need to add code to safely transition S_DAX. Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu> Reviewed-by: Jan Kara <jack@suse.cz> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.