ext4: encrypt only up to last block in ext4_bio_write_page()

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.

Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 40ee33df576494487bd48c5a83b09fe405b16f6f..a18a47a2a1d1ac26ed9c4bf6ce33bde942be4f1a 100644 (file)
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -467,11 +467,19 @@ 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);