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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 6B99FC433E6 for ; Tue, 16 Mar 2021 19:29:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3258464FBD for ; Tue, 16 Mar 2021 19:29:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240477AbhCPT3M (ORCPT ); Tue, 16 Mar 2021 15:29:12 -0400 Received: from casper.infradead.org ([90.155.50.34]:59198 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240460AbhCPT2c (ORCPT ); Tue, 16 Mar 2021 15:28:32 -0400 X-Greylist: delayed 1253 seconds by postgrey-1.27 at vger.kernel.org; Tue, 16 Mar 2021 15:28:32 EDT DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=a+0vOSKhaFyvZ89GWuo3huxCDtu4K+x/0Wx25fIk42U=; b=jt05dHVt86rt/fg//Nd6cO5DVR 1G3IYjzp8NmtC3ioV47ZhvEwsVO11uGSJs4d6RoCIe/BcJ/xaLebCaSFyAaRPStsmz5aU3Z0/tTjC i2D7OiygMf3930tWOiLJfa4OdaVC2y2beYtlioE8Uk6T6FlNM29ZnQ8CELwXsvZbjeDKpcF11lhjh fxWoakrSFVJne6ZMS7JRWMDDO0gZO7qnjt44rLaOVOowDrCMDY6XwMEq7NZ3RYp/xCyc/iArZ07hH 6dhLiP7mVnkIYcFu7Y/+Bu3gq9ijgn43hiplbkX3GbtJmVLUTJ4dq2oS7nwTEqPpCsU1+5QgJ/TDr 8DraJk1w==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1lMF23-000UZR-6Y; Tue, 16 Mar 2021 19:07:08 +0000 Date: Tue, 16 Mar 2021 19:07:07 +0000 From: Matthew Wilcox To: David Howells Cc: Trond Myklebust , Anna Schumaker , Steve French , Dominique Martinet , Linus Torvalds , Christoph Hellwig , Alexander Viro , linux-mm@kvack.org, linux-cachefs@redhat.com, linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org, linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org, v9fs-developer@lists.sourceforge.net, linux-fsdevel@vger.kernel.org, Jeff Layton , David Wysochanski , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache Message-ID: <20210316190707.GD3420@casper.infradead.org> References: <161539526152.286939.8589700175877370401.stgit@warthog.procyon.org.uk> <161539528910.286939.1252328699383291173.stgit@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <161539528910.286939.1252328699383291173.stgit@warthog.procyon.org.uk> Precedence: bulk List-ID: X-Mailing-List: ceph-devel@vger.kernel.org On Wed, Mar 10, 2021 at 04:54:49PM +0000, David Howells wrote: > Add a function, unlock_page_private_2(), to unlock PG_private_2 analogous > to that of PG_lock. Add a kerneldoc banner to that indicating the example > usage case. This isn't a problem with this patch per se, but I'm concerned about private2 and expected page refcounts. static inline int is_page_cache_freeable(struct page *page) { /* * A freeable page cache page is referenced only by the caller * that isolated the page, the page cache and optional buffer * heads at page->private. */ int page_cache_pins = thp_nr_pages(page); return page_count(page) - page_has_private(page) == 1 + page_cache_pins; } static inline int page_has_private(struct page *page) { return !!(page->flags & PAGE_FLAGS_PRIVATE); } #define PAGE_FLAGS_PRIVATE \ (1UL << PG_private | 1UL << PG_private_2) So ... a page with both flags cleared should have a refcount of N. A page with one or both flags set should have a refcount of N+1. How is a poor filesystem supposed to make that true? Also btrfs has this problem since it uses private_2 for its own purposes.