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 AD1B7C54EE9 for ; Sun, 11 Sep 2022 03:51:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229808AbiIKDvc (ORCPT ); Sat, 10 Sep 2022 23:51:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48442 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229766AbiIKDv0 (ORCPT ); Sat, 10 Sep 2022 23:51:26 -0400 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA2E32F01F for ; Sat, 10 Sep 2022 20:51:23 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=fzaCnq+vbd/vpfxECV4iLovfuzdgvAIS76l3dqNIxOY=; b=Dkz79oZU4IJip9NpZkaenXbB9+ y7M8c5/r6hZZUtzme3dRAH8gRSMIalqgOT7fNwqbOqR7gbBEQ+d3YavkjGkmnZMzL7jReKOqfPf1N ycacnMp0BtEbUGTZSsV+LYg1DVX3UYC/pW5fVFj07nGTwc/cv10EjicJGw4Y0kqbbTQ5wZJ79L9VG cWSfPlMioNnZoxBsSBM38mxdufgZs+sO6j8aGrUQkyOH31R1OzQx8K2G7cSKxHaT7IZRjYsqSfDbR ag2uWSTatGHNYicvMsda6O2LswMv0ReOzuNwpVJHqHtFasa+mxaDSnK4GQhHlB9PFBhXYnL5P538l ynwSOhCQ==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.95 #2 (Red Hat Linux)) id 1oXDzx-00Eaaa-F2; Sun, 11 Sep 2022 03:51:09 +0000 Date: Sun, 11 Sep 2022 04:51:09 +0100 From: Al Viro To: Chuck Lever III Cc: Benjamin Coddington , Olga Kornievskaia , Jeff Layton , Linux NFS Mailing List Subject: Re: Is this nfsd kernel oops known? Message-ID: References: <11BEA7FE-4CBC-4E5C-9B68-A0310CF1F3BE@oracle.com> <25AF9743-A2A2-4AFE-9123-BAD3C8F17655@redhat.com> <9D6CDF68-6B12-44DE-BC01-3BD0251E7F94@oracle.com> <5FF21605-6F1E-4DF1-A141-F86263CA579F@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5FF21605-6F1E-4DF1-A141-F86263CA579F@oracle.com> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org On Sat, Sep 10, 2022 at 10:35:52PM +0000, Chuck Lever III wrote: > > > > On Sep 10, 2022, at 6:13 PM, Al Viro wrote: > > > > On Sat, Sep 10, 2022 at 09:21:11PM +0000, Chuck Lever III wrote: > > > >> It's also possible that recent simplifications I've done to the splice > >> read actor accidentally removed the ability to deal with compound pages. > >> You might want to review the commit history of nfsd_splice_actor() to > >> see if there is an historic version that would behave correctly with > >> the new copy_page_to_iter(). > > > > Nah, that's unrelated... > > > >> Is the need to deal with CompoundPage documented somewhere? If not, > >> perhaps nfsd_splice_actor() could mention it so that overzealous > >> maintainers don't break it again. > > > >>> + struct page *page = buf->page; // may be a compound one > > > > Does that qualify? ;-) > > Well, no, since you just added it :-) I meant pre-existing > documentation of the API. I take your remark as polite > encouragement to go and look for it, because this is an > area where I need deeper understanding. Not really - quality of documentation aside, it's a combination of splice from sockets being capable of stuffing skb fragments into destination pipe and skb allocations using compound pages. E.g. AF_UNIX sendmsg() on a large datagram will result in that. socketpair(), then such sendmsg() on one end, splice() from another and there you go - references to compound pages ending up in pipe buffers... nfsd_splice_read() does file-to-pipe splice (into internal pipe) + feeding the contents of that pipe into nfsd_splice_actor(); the new part here is that file-to-pipe splice from regular file can end up with the thing that had always been possible for file-to-pipe splice from sockets...