From patchwork Tue Sep 21 14:32:59 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "J. Bruce Fields" X-Patchwork-Id: 12507977 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-17.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D8469C433EF for ; Tue, 21 Sep 2021 14:33:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BD43860F9E for ; Tue, 21 Sep 2021 14:33:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233647AbhIUOea (ORCPT ); Tue, 21 Sep 2021 10:34:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37294 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233635AbhIUOea (ORCPT ); Tue, 21 Sep 2021 10:34:30 -0400 Received: from fieldses.org (fieldses.org [IPv6:2600:3c00:e000:2f7::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C71C0C061574 for ; Tue, 21 Sep 2021 07:33:01 -0700 (PDT) Received: by fieldses.org (Postfix, from userid 2815) id 728211512; Tue, 21 Sep 2021 10:32:59 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.11.0 fieldses.org 728211512 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fieldses.org; s=default; t=1632234779; bh=GWGVSENLrMQ4sSL6QoXqiZ9vygYwt/3pMPujuZsJLX4=; h=Date:To:Subject:From:From; b=gSjegIXnzrbMWKQxcOyI0JEOPtZ2kcnCPlqOP9P3Qbis7Dcw/LJ9XLAOVqoJBuH3Z 483zY4P+0Xcn1qeygGG/+ivPgxdQhFPOk7tBysY8dM8FvmG1ePzQmI9xK/l+r0fV/T 7qFSDroBfZ0pbXbQ9rer7Qe7VHaKd+eSRys01HtY= Date: Tue, 21 Sep 2021 10:32:59 -0400 To: linux-nfs@vger.kernel.org Subject: [PATCH] nfs: reexport documentation Message-ID: <20210921143259.GB21704@fieldses.org> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) From: bfields@fieldses.org (J. Bruce Fields) Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org From: "J. Bruce Fields" We've supported reexport for a while but documentation is limited. This is mainly a simplified version of the text I wrote for the linux-nfs wiki at https://wiki.linux-nfs.org/wiki/index.php/NFS_re-export. Signed-off-by: J. Bruce Fields --- Documentation/filesystems/nfs/index.rst | 1 + Documentation/filesystems/nfs/reexport.rst | 110 +++++++++++++++++++++ 2 files changed, 111 insertions(+) create mode 100644 Documentation/filesystems/nfs/reexport.rst diff --git a/Documentation/filesystems/nfs/index.rst b/Documentation/filesystems/nfs/index.rst index 65805624e39b..288d8ddb2bc6 100644 --- a/Documentation/filesystems/nfs/index.rst +++ b/Documentation/filesystems/nfs/index.rst @@ -11,3 +11,4 @@ NFS rpc-server-gss nfs41-server knfsd-stats + reexport diff --git a/Documentation/filesystems/nfs/reexport.rst b/Documentation/filesystems/nfs/reexport.rst new file mode 100644 index 000000000000..892cb1e9c45c --- /dev/null +++ b/Documentation/filesystems/nfs/reexport.rst @@ -0,0 +1,110 @@ +Reexporting NFS filesystems +=========================== + +Overview +-------- + +It is possible to reexport an NFS filesystem over NFS. However, this +feature comes with a number of limitations. Before trying it, we +recommend some careful research to determine wether it will work for +your purposes. + +A discussion of current known limitations follows. + +"fsid=" required, crossmnt broken +--------------------------------- + +We require the "fsid=" export option on any reexport of an NFS +filesystem. + +The "crossmnt" export option does not work in the reexport case. + +Reboot recovery +--------------- + +The NFS protocol's normal reboot recovery mechanisms don't work for the +case when the reexport server reboots. Clients will lose any locks +they held before the reboot, and further IO will result in errors. +Closing and reopening files should clear the errors. + +Filehandle limits +----------------- + +If the original server uses an X byte filehandle for a given object, the +reexport server's filehandle for the reexported object will be X+22 +bytes, rounded up to the nearest multiple of four bytes. + +The result must fit into the RFC-mandated filehandle size limits: + ++-------+-----------+ +| NFSv2 | 32 bytes | ++-------+-----------+ +| NFSv3 | 64 bytes | ++-------+-----------+ +| NFSv4 | 128 bytes | ++-------+-----------+ + +So, for example, you will only be able to reexport a filesystem over +NFSv2 if the original server gives you filehandles that fit in 10 +bytes--which is unlikely. + +In general there's no way to know the maximum filehandle size given out +by an NFS server without asking the server vendor. + +But the following table gives a few examples. The first column is the +typical length of the filehandle from a Linux server exporting the given +filesystem, the second is the length after that nfs export is reexported +by another Linux host: + ++--------+-------------------+----------------+ +| | filehandle length | after reexport | ++========+===================+================+ +| ext4: | 28 bytes | 52 bytes | ++--------+-------------------+----------------+ +| xfs: | 32 bytes | 56 bytes | ++--------+-------------------+----------------+ +| btrfs: | 40 bytes | 64 bytes | ++--------+-------------------+----------------+ + +All will therefore fit in an NFSv3 or NFSv4 filehandle after reexport, +but none are reexportable over NFSv2. + +Linux server filehandles are a bit more complicated than this, though; +for example: + + - The (non-default) "subtreecheck" export option generally + requires another 4 to 8 bytes in the filehandle. + - If you export a subdirectory of a filesystem (instead of + exporting the filesystem root), that also usually adds 4 to 8 + bytes. + - If you export over NFSv2, knfsd usually uses a shorter + filesystem identifier that saves 8 bytes. + - The root directory of an export uses a filehandle that is + shorter. + +As you can see, the 128-byte NFSv4 filehandle is large enough that +you're unlikely to have trouble using NFSv4 to reexport any filesystem +exported from a Linux server. In general, if the original server is +something that also supports NFSv3, you're *probably* OK. Re-exporting +over NFSv3 may be dicier, and reexporting over NFSv2 will probably +never work. + +For more details of Linux filehandle structure, the best reference is +the source code and comments; see in particular: + + - include/linux/exportfs.h:enum fid_type + - include/uapi/linux/nfsd/nfsfh.h:struct nfs_fhbase_new + - fs/nfsd/nfsfh.c:set_version_and_fsid_type + - fs/nfs/export.c:nfs_encode_fh + +Open DENY bits ignored +---------------------- + +NFS since NFSv4 supports ALLOW and DENY bits taken from Windows, which +allow you, for example, to open a file in a mode which forbids other +read opens or write opens. The Linux client doesn't use them, and the +server's support has always been incomplete: they are enforced only +against other NFS users, not against processes accessing the exported +filesystem locally. A reexport server will also not pass them along to +the original server, so they will not be enforced between clients of +different reexport servers.