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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 BEBAAC43462 for ; Thu, 8 Apr 2021 16:45:43 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 29EC7610D1 for ; Thu, 8 Apr 2021 16:45:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 29EC7610D1 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id A5F696B0083; Thu, 8 Apr 2021 12:45:42 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A12006B0085; Thu, 8 Apr 2021 12:45:42 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 9281B6B0087; Thu, 8 Apr 2021 12:45:42 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0075.hostedemail.com [216.40.44.75]) by kanga.kvack.org (Postfix) with ESMTP id 7400C6B0083 for ; Thu, 8 Apr 2021 12:45:42 -0400 (EDT) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 0A4B66D8F for ; Thu, 8 Apr 2021 16:45:42 +0000 (UTC) X-FDA: 78009776124.06.A278DFC Received: from zeniv-ca.linux.org.uk (zeniv-ca.linux.org.uk [142.44.231.140]) by imf30.hostedemail.com (Postfix) with ESMTP id ABA05E000104 for ; Thu, 8 Apr 2021 16:45:34 +0000 (UTC) Received: from viro by zeniv-ca.linux.org.uk with local (Exim 4.94 #2 (Red Hat Linux)) id 1lUXmj-003kcY-LV; Thu, 08 Apr 2021 16:45:37 +0000 Date: Thu, 8 Apr 2021 16:45:37 +0000 From: Al Viro To: Daniel Xu Cc: bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, jolsa@kernel.org, hannes@cmpxchg.org, yhs@fb.com Subject: Re: [RFC bpf-next 1/1] bpf: Introduce iter_pagecache Message-ID: References: <22bededbd502e0df45326a54b3056941de65a101.1617831474.git.dxu@dxuuu.xyz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <22bededbd502e0df45326a54b3056941de65a101.1617831474.git.dxu@dxuuu.xyz> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: ABA05E000104 X-Stat-Signature: ty5uea4otks1k96c39uipwpguoc7j5w5 Received-SPF: none (ftp.linux.org.uk>: No applicable sender policy available) receiver=imf30; identity=mailfrom; envelope-from=""; helo=zeniv-ca.linux.org.uk; client-ip=142.44.231.140 X-HE-DKIM-Result: none/none X-HE-Tag: 1617900334-572781 X-Bogosity: Ham, tests=bogofilter, spamicity=0.003484, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Apr 07, 2021 at 02:46:11PM -0700, Daniel Xu wrote: > +static void fini_seq_pagecache(void *priv_data) > +{ > + struct bpf_iter_seq_pagecache_info *info = priv_data; > + struct radix_tree_iter iter; > + struct super_block *sb; > + void **slot; > + > + radix_tree_for_each_slot(slot, &info->superblocks, &iter, 0) { > + sb = (struct super_block *)iter.index; > + atomic_dec(&sb->s_active); > + radix_tree_delete(&info->superblocks, iter.index); > + } ... and if in the meanwhile all other contributors to ->s_active have gone away, that will result in...? IOW, NAK. The objects you are playing with have non-trivial lifecycle and poking into the guts of data structures without bothering to understand it is not a good idea. Rule of the thumb: if your code ends up using fields that are otherwise handled by a small part of codebase, the odds are that you need to be bloody careful. In particular, ->ns_lock has 3 users - all in fs/namespace.c. ->list/->mnt_list: all users in fs/namespace.c and fs/pnode.c. ->s_active: majority in fs/super.c, with several outliers in filesystems and safety of those is not trivial. Any time you see that kind of pattern, you are risking to reprise a scene from The Modern Times - the one with Charlie taking a trip through the guts of machinery.