From patchwork Tue Nov 27 16:00:10 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Johannes Thumshirn X-Patchwork-Id: 10700749 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id E708D13AD for ; Tue, 27 Nov 2018 16:00:25 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id CE3502BEE0 for ; Tue, 27 Nov 2018 16:00:25 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id CC3992BED8; Tue, 27 Nov 2018 16:00:25 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 754BC2BEE0 for ; Tue, 27 Nov 2018 16:00:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730434AbeK1C6n (ORCPT ); Tue, 27 Nov 2018 21:58:43 -0500 Received: from mx2.suse.de ([195.135.220.15]:54480 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730421AbeK1C6n (ORCPT ); Tue, 27 Nov 2018 21:58:43 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id BA9BAAF8A for ; Tue, 27 Nov 2018 16:00:20 +0000 (UTC) From: Johannes Thumshirn To: David Sterba Cc: Linux BTRFS Mailinglist , Johannes Thumshirn Subject: [PATCH 3/3] btrfs: document extent mapping assumptions in checksum Date: Tue, 27 Nov 2018 17:00:10 +0100 Message-Id: <20181127160010.18123-4-jthumshirn@suse.de> X-Mailer: git-send-email 2.16.4 In-Reply-To: <20181127160010.18123-1-jthumshirn@suse.de> References: <20181127160010.18123-1-jthumshirn@suse.de> Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Document why map_private_extent_buffer() cannot return '1' (i.e. the map spans two pages) for the csum_tree_block() case. The current algorithm for detecting a page boundary crossing in map_private_extent_buffer() will return a '1' *IFF* the product of the extent buffer's offset in the page + the offset passed in by csum_tree_block() and the minimal length passed in by csum_tree_block() - 1 are bigger than PAGE_SIZE. We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32 and the current extent buffer allocator always guarantees page aligned extends, so the above condition can't be true. Signed-off-by: Johannes Thumshirn Reviewed-by: Nikolay Borisov --- fs/btrfs/disk-io.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 4bc270ef29b4..14d355d0cb7a 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info, len = buf->len - offset; while (len > 0) { + /* + * Note: we don't need to check for the err == 1 case here, as + * with the given combination of 'start = BTRFS_CSUM_SIZE (32)' + * and 'min_len = 32' and the currently implemented mapping + * algorithm we cannot cross a page boundary. + */ err = map_private_extent_buffer(buf, offset, 32, &kaddr, &map_start, &map_len); if (err)