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fscrypt: update documentation for direct I/O support 

Now that direct I/O is supported on encrypted files in some cases,
document what these cases are.

Link: https://lore.kernel.org/r/20220128233940.79464-6-ebiggers@kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Eric Biggers 2022-01-28 15:39:40 -08:00 committed by Jaegeuk Kim
parent 95f58823a6
commit f78d4e6626
1 changed files with 23 additions and 2 deletions
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25
Documentation/filesystems/fscrypt.rst
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@ -1047,8 +1047,8 @@ astute users may notice some differences in behavior:
may be used to overwrite the source files but isn't guaranteed to be
effective on all filesystems and storage devices.
- Direct I/O is not supported on encrypted files. Attempts to use
direct I/O on such files will fall back to buffered I/O.
- Direct I/O is supported on encrypted files only under some
circumstances. For details, see `Direct I/O support`_.
- The fallocate operations FALLOC_FL_COLLAPSE_RANGE and
FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will
@ -1179,6 +1179,27 @@ Inline encryption doesn't affect the ciphertext or other aspects of
the on-disk format, so users may freely switch back and forth between
using "inlinecrypt" and not using "inlinecrypt".
Direct I/O support
==================
For direct I/O on an encrypted file to work, the following conditions
must be met (in addition to the conditions for direct I/O on an
unencrypted file):
* The file must be using inline encryption. Usually this means that
the filesystem must be mounted with ``-o inlinecrypt`` and inline
encryption hardware must be present. However, a software fallback
is also available. For details, see `Inline encryption support`_.
* The I/O request must be fully aligned to the filesystem block size.
This means that the file position the I/O is targeting, the lengths
of all I/O segments, and the memory addresses of all I/O buffers
must be multiples of this value. Note that the filesystem block
size may be greater than the logical block size of the block device.
If either of the above conditions is not met, then direct I/O on the
encrypted file will fall back to buffered I/O.
Implementation details
======================