From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 283C8EB64D9 for ; Mon, 19 Jun 2023 11:03:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232457AbjFSLDa (ORCPT ); Mon, 19 Jun 2023 07:03:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33564 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232424AbjFSLC6 (ORCPT ); Mon, 19 Jun 2023 07:02:58 -0400 Received: from out30-113.freemail.mail.aliyun.com (out30-113.freemail.mail.aliyun.com [115.124.30.113]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 207701BC3; Mon, 19 Jun 2023 04:01:53 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045170;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0VlV8FlL_1687172510; Received: from localhost(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0VlV8FlL_1687172510) by smtp.aliyun-inc.com; Mon, 19 Jun 2023 19:01:51 +0800 From: Jingbo Xu To: hsiangkao@linux.alibaba.com, chao@kernel.org, linux-erofs@lists.ozlabs.org Cc: huyue2@coolpad.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] erofs: add documentation for long xattr name prefixes feature Date: Mon, 19 Jun 2023 19:01:50 +0800 Message-Id: <20230619110150.107958-1-jefflexu@linux.alibaba.com> X-Mailer: git-send-email 2.19.1.6.gb485710b MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Add documentation for the long xattr name prefixes feature, which has landed upstream since v6.4. Signed-off-by: Jingbo Xu --- Documentation/filesystems/erofs.rst | 47 +++++++++++++++++++++++++++++ 1 file changed, 47 insertions(+) diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst index 4654ee57c1d5..d3408d25165e 100644 --- a/Documentation/filesystems/erofs.rst +++ b/Documentation/filesystems/erofs.rst @@ -328,3 +328,50 @@ but it's easy to know the size of such pcluster is 1 lcluster as well. Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, therefore it can be used for compressed data deduplication. + +Long xattr name prefixes +------------------------ +In use cases together with overlayfs like Composefs model, there are xattrs with +diverse xattr values but only a few common xattr names (trusted.overlay.redirect, +trusted.overlay.digest, and maybe more in the future). The existing predefined +prefixes mechanism, e.g. {.e_name_index = EROFS_XATTR_INDEX_TRUSTED, +.e_name = "overlay.redirect"} for "trusted.overlay.redirect", works inefficiently +in both image size and runtime performance in such scenarios. + +The long xattr name prefixes feature is introduced (since Linux v6.4) to address +this issue. The general idea is that, apart from the existing predefined +prefixes, the xattr entry could also refer to user specified long xattr name +prefixes, e.g. "trusted.overlay." in above described use cases. + +When referring to a long xattr name prefix, the highest bit (bit 7) of +erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole +represents the index of the referred long name prefix among all long xattr name +prefixes. Besides only the trailing part of the xattr name apart from the long +xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if +the xattr name matches exactly as the long xattr name prefix. + +All long xattr prefixes are stored one by one in the packed inode as long as the +packed inode exists, or meta inode otherwise. The xattr_prefix_count (of +on-disk superblock) represents the total number of the long xattr name prefixes, +while (xattr_prefix_start * 4) indicates the start offset of long name prefixes +in the packed/meta inode. The long xattr name prefixes feature is not used when +xattr_prefix_count is 0. + +Each long xattr name prefix (in packed/meta inode) is stored in the format: +ALIGN({__le16 len, data}, 4), where len represents the total size of following +data. The data part, describing one long xattr name prefixes record, is +represented by 'struct erofs_xattr_long_prefix', where base_index represents the +index of the referred predefined short xattr name prefix, e.g. +EROFS_XATTR_INDEX_TRUSTED for "trusted.overlay." long name prefix, while the +infix string represents the string after stripping the short prefix, e.g. +"overlay." for the above example. + +:: + + |-> aligned with 4 bytes |-> aligned with 4 bytes + _________________________________________________________________________________ + | .. | len | base_index | infix string | .. | len | base_index | infix string | .. + |____|_____|____________|______________|____|_____|____________|______________|___ + ^ |<-- xattr_long_prefix -->| |<-- xattr_long_prefix -->| + . + + (xattr_prefix_start * 4) offset -- 2.19.1.6.gb485710b