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 9BD3BC43334 for ; Fri, 8 Jul 2022 18:11:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238736AbiGHSLH (ORCPT ); Fri, 8 Jul 2022 14:11:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33372 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238232AbiGHSLG (ORCPT ); Fri, 8 Jul 2022 14:11:06 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 16AA0564FF for ; Fri, 8 Jul 2022 11:11:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1657303864; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=9lAFzX2dtGRg3YSEBVWrz3UBy/sUXecMhW9ErT0p4M0=; b=AgNABZ+Jfbc1RtpJfpZQQvrVF3xLDH09UvfABwsH+DhqVH7aeHzW9Izro2kcSTXk7EHSZn gbtj31IqsVzuOrS6jOOi4hHjfGV+UsIQfhuWeSjpClCrSJYrwF67pJRb+hgPioEgbl2BPG Q9/My5hCBaVSgvf5vOQZ2iqy7BoGV6g= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-558-HL9f52CWNgqLPspSr_kd4w-1; Fri, 08 Jul 2022 14:10:54 -0400 X-MC-Unique: HL9f52CWNgqLPspSr_kd4w-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 31216811E87; Fri, 8 Jul 2022 18:10:54 +0000 (UTC) Received: from [172.16.176.1] (unknown [10.22.48.8]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 118372026D64; Fri, 8 Jul 2022 18:10:51 +0000 (UTC) From: "Benjamin Coddington" To: "Eric Dumazet" Cc: "Guillaume Nault" , "David S. Miller" , "Jakub Kicinski" , "Paolo Abeni" , netdev , "Chuck Lever" , "Jeff Layton" , "Trond Myklebust" , "Anna Schumaker" , "Steve French" , "Josef Bacik" , "Scott Mayhew" , "Tejun Heo" Subject: Re: [RFC net] Should sk_page_frag() also look at the current GFP context? Date: Fri, 08 Jul 2022 14:10:50 -0400 Message-ID: <429C561E-2F85-4DB5-993C-B2DD4E575BF0@redhat.com> In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; format=flowed X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 7 Jul 2022, at 12:29, Eric Dumazet wrote: > On Fri, Jul 1, 2022 at 8:41 PM Guillaume Nault > wrote: >> >> I'm investigating a kernel oops that looks similar to >> 20eb4f29b602 ("net: fix sk_page_frag() recursion from memory >> reclaim") >> and dacb5d8875cc ("tcp: fix page frag corruption on page fault"). >> >> This time the problem happens on an NFS client, while the previous >> bzs >> respectively used NBD and CIFS. While NBD and CIFS clear __GFP_FS in >> their socket's ->sk_allocation field (using GFP_NOIO or GFP_NOFS), >> NFS >> leaves sk_allocation to its default value since commit a1231fda7e94 >> ("SUNRPC: Set memalloc_nofs_save() on all rpciod/xprtiod jobs"). >> >> To recap the original problems, in commit 20eb4f29b602 and >> dacb5d8875cc, >> memory reclaim happened while executing tcp_sendmsg_locked(). The >> code >> path entered tcp_sendmsg_locked() recursively as pages to be >> reclaimed >> were backed by files on the network. The problem was that both the >> outer and the inner tcp_sendmsg_locked() calls used >> current->task_frag, >> thus leaving it in an inconsistent state. The fix was to use the >> socket's ->sk_frag instead for the file system socket, so that the >> inner and outer calls wouln't step on each other's toes. >> >> But now that NFS doesn't modify ->sk_allocation anymore, >> sk_page_frag() >> sees sunrpc sockets as plain TCP ones and returns ->task_frag in the >> inner tcp_sendmsg_locked() call. >> >> Also it looks like the trend is to avoid GFS_NOFS and GFP_NOIO and >> use >> memalloc_no{fs,io}_save() instead. So maybe other network file >> systems >> will also stop setting ->sk_allocation in the future and we should >> teach sk_page_frag() to look at the current GFP flags. Or should we >> stick to ->sk_allocation and make NFS drop __GFP_FS again? >> >> Signed-off-by: Guillaume Nault > > Can you provide a Fixes: tag ? > >> --- >> include/net/sock.h | 8 ++++++-- >> 1 file changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/include/net/sock.h b/include/net/sock.h >> index 72ca97ccb460..b934c9851058 100644 >> --- a/include/net/sock.h >> +++ b/include/net/sock.h >> @@ -46,6 +46,7 @@ >> #include >> #include /* struct sk_buff */ >> #include >> +#include >> #include >> #include >> #include >> @@ -2503,14 +2504,17 @@ static inline void >> sk_stream_moderate_sndbuf(struct sock *sk) >> * socket operations and end up recursing into sk_page_frag() >> * while it's already in use: explicitly avoid task page_frag >> * usage if the caller is potentially doing any of them. >> - * This assumes that page fault handlers use the GFP_NOFS flags. >> + * This assumes that page fault handlers use the GFP_NOFS flags >> + * or run under memalloc_nofs_save() protection. >> * >> * Return: a per task page_frag if context allows that, >> * otherwise a per socket one. >> */ >> static inline struct page_frag *sk_page_frag(struct sock *sk) >> { >> - if ((sk->sk_allocation & (__GFP_DIRECT_RECLAIM | >> __GFP_MEMALLOC | __GFP_FS)) == >> + gfp_t gfp_mask = current_gfp_context(sk->sk_allocation); > > This is slowing down TCP sendmsg() fast path, reading current->flags, > possibly cold value. True - current->flags is pretty distant from current->task_frag. > I would suggest using one bit in sk, close to sk->sk_allocation to > make the decision, > instead of testing sk->sk_allocation for various flags. > > Not sure if we have available holes. Its looking pretty packed on my build.. the nearest hole is 5 cachelines away. It'd be nice to allow network filesystem to use task_frag when possible. If we expect sk_page_frag() to only return task_frag once per call stack, then can we simply check it's already in use, perhaps by looking at the size field? Or maybe can we set sk_allocation early from current_gfp_context() outside the fast path? Ben