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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no 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 9CDF6C4724C for ; Thu, 30 Apr 2020 21:40:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B3322064C for ; Thu, 30 Apr 2020 21:40:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588282823; bh=mJs5hluqduo9dtI8ms49qYStG1gIgUgTJaDMGc0TwsY=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=r3hWvcWOaUkAuwLDbVrGLG7SJgAsNqSDkvg6+mYLBQ/qkU3YiWwuJXxgN+7H703A9 fuc9METYMIxEGYuXjFi5vJj79S0MuFq8sbefDIneYeR0jm17UucqMp/xAbWhOBDMH8 tPNYOC4gqrhgAQSijyEkAkkqCAuDJudu7AtGGuQc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726926AbgD3VkU (ORCPT ); Thu, 30 Apr 2020 17:40:20 -0400 Received: from mail.kernel.org ([198.145.29.99]:48196 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726447AbgD3VkU (ORCPT ); Thu, 30 Apr 2020 17:40:20 -0400 Received: from localhost.localdomain (c-73-231-172-41.hsd1.ca.comcast.net [73.231.172.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 872792064C; Thu, 30 Apr 2020 21:40:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588282819; bh=mJs5hluqduo9dtI8ms49qYStG1gIgUgTJaDMGc0TwsY=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=bC8IorIvBOl0MDL52/kEKkObD8oM2TuQj0h4qMWCgw91RtBB3hNmYby7KsliyZY/M zZl0imt5lRxodssgDt7UYZkzSoJBk7pK5roliimn5FjgWANZtHDXpfsNqtHfYlO33c JCGyBuuntmyuEFjLJdo7KY8mjVnUhjBCyuK81QjY= Date: Thu, 30 Apr 2020 14:40:18 -0700 From: Andrew Morton To: fdmanana@kernel.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, dennis@kernel.org, tj@kernel.org, cl@linux.com, linux-btrfs@vger.kernel.org, Filipe Manana Subject: Re: [PATCH] percpu: make pcpu_alloc() aware of current gfp context Message-Id: <20200430144018.c855f031b321d68e5c89b30c@linux-foundation.org> In-Reply-To: <20200430164356.15543-1-fdmanana@kernel.org> References: <20200430164356.15543-1-fdmanana@kernel.org> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Thu, 30 Apr 2020 17:43:56 +0100 fdmanana@kernel.org wrote: > From: Filipe Manana > > Since 5.7-rc1, on btrfs we have a percpu counter initialization for which > we always pass a GFP_KERNEL gfp_t argument (this happens since commit > 2992df73268f78 ("btrfs: Implement DREW lock")). That is safe in some > contextes but not on others where allowing fs reclaim could lead to a > deadlock because we are either holding some btrfs lock needed for a > transaction commit or holding a btrfs transaction handle open. Because > of that we surround the call to the function that initializes the percpu > counter with a NOFS context using memalloc_nofs_save() (this is done at > btrfs_init_fs_root()). > > However it turns out that this is not enough to prevent a possible > deadlock because percpu_alloc() determines if it is in an atomic context > by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this > case) and it is not aware that a NOFS context is set. Because it thinks > it is in a non atomic context it locks the pcpu_alloc_mutex, which can > result in a btrfs deadlock when pcpu_balance_workfn() is running, has > acquired that mutex and is waiting for reclaim, while the btrfs task that > called percpu_counter_init() (and therefore percpu_alloc()) is holding > either the btrfs commit_root semaphore or a transaction handle (done at > fs/btrfs/backref.c:iterate_extent_inodes()), which prevents reclaim from > finishing as an attempt to commit the current btrfs transaction will > deadlock. > Patch looks good and seems sensible, thanks. But why did btrfs use memalloc_nofs_save()/restore() rather than s/GFP_KERNEL/GFP_NOFS/?