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=DKIM_INVALID,DKIM_SIGNED, 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 9B7BCC4332D for ; Fri, 20 Mar 2020 18:25:07 +0000 (UTC) Received: from lists.sourceforge.net (lists.sourceforge.net [216.105.38.7]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 7078C20786; Fri, 20 Mar 2020 18:25:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=sourceforge.net header.i=@sourceforge.net header.b="FTDRPf3Z"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=sf.net header.i=@sf.net header.b="mHdVIjHN"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="w2Kwf2qr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7078C20786 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linux-f2fs-devel-bounces@lists.sourceforge.net Received: from [127.0.0.1] (helo=sfs-ml-4.v29.lw.sourceforge.com) by sfs-ml-4.v29.lw.sourceforge.com with esmtp (Exim 4.90_1) (envelope-from ) id 1jFMKP-0004we-5w; Fri, 20 Mar 2020 18:25:05 +0000 Received: from [172.30.20.202] (helo=mx.sourceforge.net) by sfs-ml-4.v29.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jFMKO-0004wQ-4M for linux-f2fs-devel@lists.sourceforge.net; Fri, 20 Mar 2020 18:25:04 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sourceforge.net; s=x; 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:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=XJO3xoHBsLe/dFCGylNOvGE6N3LikSbsbgNkZGMHxeU=; b=FTDRPf3Z4dRFgaGFwzHoSrMV9A Oa3cmcCM28mxymJ1Th8vZkrKsD8Cu9r+s9m3fzOIw+5xN7wjPcFqlVgjtKr+K+zvfQQZUplMB1lFh Yu2g8KjKeb8rAVWMjk+e0wxqwsdHuwdW4GYSTZ4K+XIaEHMwi/SVl3M2tSsoezzdf/zw=; DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=sf.net; s=x ; 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:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Id:List-Help:List-Unsubscribe:List-Subscribe: List-Post:List-Owner:List-Archive; bh=XJO3xoHBsLe/dFCGylNOvGE6N3LikSbsbgNkZGMHxeU=; b=mHdVIjHNWn6peEreBuuQhukL7I zHCQYYg5PXmNBgVpNiV8DVPKCH6oEHGIxwKtEIjwzoKGW0+TBZxAUAegTx3av4sIX/0FQO6SZDzMR hcFh0g6JfQmw2vwweyiPRiXa8CbW61lYFtHwBh4lplP/DiHThTo8QRjgUlqlz91uQ57s=; Received: from mail.kernel.org ([198.145.29.99]) by sfi-mx-4.v28.lw.sourceforge.com with esmtps (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.92.2) id 1jFMKL-001cpE-Jg for linux-f2fs-devel@lists.sourceforge.net; Fri, 20 Mar 2020 18:25:04 +0000 Received: from sol.localdomain (c-107-3-166-239.hsd1.ca.comcast.net [107.3.166.239]) (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 E406F20767; Fri, 20 Mar 2020 18:24:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584728694; bh=iXk1jLxqgPXgnR9ja2pvER1tlxPb7y75DLTL+2ZEEvs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=w2Kwf2qrgq9mF3e+Y3DstRvcRM3IPcP1rBsX2IFVwGk7IR9XNsgwueVzsL3c0gBZe +MuPXs0V8pB0UpnbySniOTBsbDmUlJD9DiQwI2PQqZKV0rdBr17ai4ecQWWo63ilaO umg5VrnjKfVftfBjPZrbhNTs/LLm4csl2Am6T0gA= Date: Fri, 20 Mar 2020 11:24:52 -0700 From: Eric Biggers To: Matthew Wilcox Message-ID: <20200320182452.GF851@sol.localdomain> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> <20200320165828.GB851@sol.localdomain> <20200320173040.GB4971@bombadil.infradead.org> <20200320180017.GE851@sol.localdomain> <20200320181132.GD4971@bombadil.infradead.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200320181132.GD4971@bombadil.infradead.org> X-Headers-End: 1jFMKL-001cpE-Jg Subject: Re: [f2fs-dev] [PATCH v9 12/25] mm: Move end_index check out of readahead loop X-BeenThere: linux-f2fs-devel@lists.sourceforge.net X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: cluster-devel@redhat.com, linux-mm@kvack.org, John Hubbard , linux-kernel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, William Kucharski , linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, ocfs2-devel@oss.oracle.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net On Fri, Mar 20, 2020 at 11:11:32AM -0700, Matthew Wilcox wrote: > On Fri, Mar 20, 2020 at 11:00:17AM -0700, Eric Biggers wrote: > > On Fri, Mar 20, 2020 at 10:30:40AM -0700, Matthew Wilcox wrote: > > > 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; > > > > > > > But then if someone passes index=0 and nr_to_read=0, this underflows and the > > entire file gets read. > > nr_to_read == 0 doesn't make sense ... I thought we filtered that out > earlier, but I can't find anywhere that does that right now. I'd > rather return early from __do_page_cache_readahead() to fix that. > > > The page cache isn't actually supposed to contain a page at index ULONG_MAX, > > since MAX_LFS_FILESIZE is at most ((loff_t)ULONG_MAX << PAGE_SHIFT), right? So > > I don't think we need to worry about reading the page with index ULONG_MAX. > > I.e. I think it's fine to limit nr_to_read to 'ULONG_MAX - index', if that makes > > it easier to avoid an overflow or underflow in the next check. > > I think we can get a page at ULONG_MAX on 32-bit systems? I mean, we can buy > hard drives which are larger than 16TiB these days: > https://www.pcmag.com/news/seagate-will-ship-18tb-and-20tb-hard-drives-in-2020 > (even ignoring RAID devices) The max file size is ((loff_t)ULONG_MAX << PAGE_SHIFT) which means the maximum page *index* is ULONG_MAX - 1, not ULONG_MAX. Anyway, I think we may be making this much too complicated. How about just: pgoff_t i_nrpages = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE); if (index >= i_nrpages) return; /* Don't read past the end of the file */ nr_to_read = min(nr_to_read, i_nrpages - index); That's 2 branches instead of 4. (Note that assigning to i_nrpages can't overflow, since the max number of pages is ULONG_MAX not ULONG_MAX + 1.) - Eric _______________________________________________ Linux-f2fs-devel mailing list Linux-f2fs-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel