ext4: decrypt only the needed blocks in ext4_block_write_begin()

In ext4_block_write_begin(), only decrypt the blocks that actually need
to be decrypted (up to two blocks which intersect the boundaries of the
region that will be written to), rather than assuming blocksize ==
PAGE_SIZE and decrypting the whole page.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.
Signed-off-by: Chandan Rajendra <chandan@linux.ibm.com>
(EB: rebase onto previous changes, improve the commit message,
 and move the check for encrypted inode)
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/ext4/inode.c

@@ -1164,8 +1164,9 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
 	int err = 0;
 	unsigned blocksize = inode->i_sb->s_blocksize;
 	unsigned bbits;
-	struct buffer_head *bh, *head, *wait[2], **wait_bh = wait;
-	bool decrypt = false;
+	struct buffer_head *bh, *head, *wait[2];
+	int nr_wait = 0;
+	int i;
 
 	BUG_ON(!PageLocked(page));
 	BUG_ON(from > PAGE_SIZE);
@@ -1217,24 +1218,30 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
 		    !buffer_unwritten(bh) &&
 		    (block_start < from || block_end > to)) {
 			ll_rw_block(REQ_OP_READ, 0, 1, &bh);
-			*wait_bh++ = bh;
-			decrypt = IS_ENCRYPTED(inode) && S_ISREG(inode->i_mode);
+			wait[nr_wait++] = bh;
 		}
 	}
 	/*
 	 * If we issued read requests, let them complete.
 	 */
-	while (wait_bh > wait) {
-		wait_on_buffer(*--wait_bh);
-		if (!buffer_uptodate(*wait_bh))
+	for (i = 0; i < nr_wait; i++) {
+		wait_on_buffer(wait[i]);
+		if (!buffer_uptodate(wait[i]))
 			err = -EIO;
 	}
 	if (unlikely(err)) {
 		page_zero_new_buffers(page, from, to);
-	} else if (decrypt) {
-		err = fscrypt_decrypt_pagecache_blocks(page, PAGE_SIZE, 0);
-		if (err)
-			clear_buffer_uptodate(*wait_bh);
+	} else if (IS_ENCRYPTED(inode) && S_ISREG(inode->i_mode)) {
+		for (i = 0; i < nr_wait; i++) {
+			int err2;
+
+			err2 = fscrypt_decrypt_pagecache_blocks(page, blocksize,
+								bh_offset(wait[i]));
+			if (err2) {
+				clear_buffer_uptodate(wait[i]);
+				err = err2;
+			}
+		}
 	}
 
 	return err;