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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 89C31C43333 for ; Fri, 20 Mar 2020 17:30:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 608ED20722 for ; Fri, 20 Mar 2020 17:30:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="EGKIuDKu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726902AbgCTRan (ORCPT ); Fri, 20 Mar 2020 13:30:43 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:49498 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726809AbgCTRam (ORCPT ); Fri, 20 Mar 2020 13:30:42 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.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=UGITiyVmaO8X73R6wVMfMGWLW2W7neAq/9meuj1VHqo=; b=EGKIuDKu0tjZuVECGtavPDxi0D O6qg1HxEZ5HTL6SQ+HnHGiXg3uMjLMXJy/QxU+RUtVkogm3OV1OoYySDKdd0TNCl75RNBRD/yIw4o r5B74nN9dCjwBJMpXdmIQPMo0Dtd++oHnpRNcQiIMtN+2/YTHKBLp/Yz4yXCj69SdRRm0YxhO6rSh QXAUKzLbScpAtIooOgBnixq+3fxkEPx9tp3sn8jy4Vs+FM4MKzh0fM6tnnEsYlBa1E1lxlobx/VwW F43ky5cdb9YLYKc01lCcZou/2CPPrIQSZW4GwTB3CgHdAptXVKYA1oettmTxaZQrltP3u3grDMdAI dACn5xZg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jFLTk-0003g5-Sd; Fri, 20 Mar 2020 17:30:40 +0000 Date: Fri, 20 Mar 2020 10:30:40 -0700 From: Matthew Wilcox To: Eric Biggers Cc: Andrew Morton , linux-xfs@vger.kernel.org, William Kucharski , John Hubbard , linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-mm@kvack.org, ocfs2-devel@oss.oracle.com, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-btrfs@vger.kernel.org Subject: Re: [PATCH v9 12/25] mm: Move end_index check out of readahead loop Message-ID: <20200320173040.GB4971@bombadil.infradead.org> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> <20200320165828.GB851@sol.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200320165828.GB851@sol.localdomain> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Fri, Mar 20, 2020 at 09:58:28AM -0700, Eric Biggers wrote: > On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > > + /* Avoid wrapping to the beginning of the file */ > > + if (index + nr_to_read < index) > > + nr_to_read = ULONG_MAX - index + 1; > > + /* Don't read past the page containing the last byte of the file */ > > + if (index + nr_to_read >= end_index) > > + nr_to_read = end_index - index + 1; > > There seem to be a couple off-by-one errors here. Shouldn't it be: > > /* Avoid wrapping to the beginning of the file */ > if (index + nr_to_read < index) > nr_to_read = ULONG_MAX - index; I think it's right. Imagine that index is ULONG_MAX. We should read one page (the one at ULONG_MAX). That would be ULONG_MAX - ULONG_MAX + 1. > /* Don't read past the page containing the last byte of the file */ > if (index + nr_to_read > end_index) > nr_to_read = end_index - index + 1; > > I.e., 'ULONG_MAX - index' rather than 'ULONG_MAX - index + 1', so that > 'index + nr_to_read' is then ULONG_MAX rather than overflowed to 0. > > Then 'index + nr_to_read > end_index' rather 'index + nr_to_read >= end_index', > since otherwise nr_to_read can be increased by 1 rather than decreased or stay > the same as expected. Ooh, I missed the overflow case here. It should be: + if (index + nr_to_read - 1 > end_index) + nr_to_read = end_index - index + 1; Let's say index comes in at ULONG_MAX - 2, end_index is ULONG_MAX - 1 and nr_to_read is 8. The first condition triggers and nr_to_read is reduced to 3. But then the second condition wouldn't trigger because ULONG_MAX - 2 + 3 is 0. With the rewrite I have in this message, ULONG_MAX - 2 + 3 - 1 is ULONG_MAX, which is > ULONG_MAX - 1. So the condition triggers and nr_to_read becomes (ULONG_MAX - 1) - (ULONG_MAX - 2) + 1. Which is -1 + 2 + 1, which is 2. Which is the right answer because we want to read two pages; the one at ULONG_MAX - 2 and the one at ULONG_MAX - 1. Thank you!