[PATCH 13/13] ext4: encrypt only up to last block in ext4_bio_write_page()

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

As an optimization, don't encrypt blocks fully beyond i_size, since
those definitely won't need to be written out.  Also add a comment.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/ext4/page-io.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 457ddf051608f..ab843ad89df2f 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -468,11 +468,19 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
 
 	bh = head = page_buffers(page);
 
+	/*
+	 * If any blocks are being written to an encrypted file, encrypt them
+	 * into a bounce page.  For simplicity, just encrypt until the last
+	 * block which might be needed.  This may cause some unneeded blocks
+	 * (e.g. holes) to be unnecessarily encrypted, but this is rare and
+	 * can't happen in the common case of blocksize == PAGE_SIZE.
+	 */
 	if (IS_ENCRYPTED(inode) && S_ISREG(inode->i_mode) && nr_to_submit) {
 		gfp_t gfp_flags = GFP_NOFS;
+		unsigned int enc_bytes = round_up(len, i_blocksize(inode));
 
 	retry_encrypt:
-		bounce_page = fscrypt_encrypt_pagecache_blocks(page, PAGE_SIZE,
+		bounce_page = fscrypt_encrypt_pagecache_blocks(page, enc_bytes,
 							       0, gfp_flags);
 		if (IS_ERR(bounce_page)) {
 			ret = PTR_ERR(bounce_page);
-- 
2.21.0.593.g511ec345e18-goog