linux-ext4 - [PATCH v10 5/5] fscrypt: update documentation for direct I/O support

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Date:   Wed, 19 Jan 2022 23:12:15 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-fscrypt@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>
Subject: [PATCH v10 5/5] fscrypt: update documentation for direct I/O support

From: Eric Biggers <ebiggers@...gle.com>

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

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 Documentation/filesystems/fscrypt.rst | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 4d5d50dca65c6..6ccd5efb25b77 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -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
 ======================
 
-- 
2.34.1