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=-7.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 47A41C4332B for ; Fri, 20 Mar 2020 16:58:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1E2C720767 for ; Fri, 20 Mar 2020 16:58:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584723516; bh=AxLDPN4LwuxzW4xjwlSgsVf6rtc0JFVKxbo5//wmPgc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=HabWpcltUBgYzjwNKfmioIHC3F5eAwXXeSC0fvBzbAc4gKFxk16tJ8QtmhxXxw71Z 8GsBPL+FoOZdhMyt6z0PIvk9C6f/1k2N6MxQbU5sfcQuFIQt4YiRNDLPmJn/UJytU1 poFoDXJFs6DPTEF/PN9mc68CDoRiWA+TtpmBXFAo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727400AbgCTQ6b (ORCPT ); Fri, 20 Mar 2020 12:58:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:38696 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727101AbgCTQ6b (ORCPT ); Fri, 20 Mar 2020 12:58:31 -0400 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 6434C20753; Fri, 20 Mar 2020 16:58:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584723510; bh=AxLDPN4LwuxzW4xjwlSgsVf6rtc0JFVKxbo5//wmPgc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=cRbat8REA7EmmJnmLCrWxRoNW5LgyCKm+4EsNDf5E4lY0nLcYrSE6Jyr+LkCnKmyi n3W2x46nv8ZHmzqeZMc86Mj56kXxAhIEwG1PHXCTucYL5iivPYgJLLRVtDE0DPEV3X iA5d4+HsRZ/BNe0ydHe/SfLeiRxRKGyEVqIcRkok= Date: Fri, 20 Mar 2020 09:58:28 -0700 From: Eric Biggers To: Matthew Wilcox 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: <20200320165828.GB851@sol.localdomain> References: <20200320142231.2402-1-willy@infradead.org> <20200320142231.2402-13-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200320142231.2402-13-willy@infradead.org> Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri, Mar 20, 2020 at 07:22:18AM -0700, Matthew Wilcox wrote: > From: "Matthew Wilcox (Oracle)" > > By reducing nr_to_read, we can eliminate this check from inside the loop. > > Signed-off-by: Matthew Wilcox (Oracle) > Reviewed-by: John Hubbard > Reviewed-by: William Kucharski > --- > mm/readahead.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/readahead.c b/mm/readahead.c > index d01531ef9f3c..a37b68f66233 100644 > --- a/mm/readahead.c > +++ b/mm/readahead.c > @@ -167,8 +167,6 @@ void __do_page_cache_readahead(struct address_space *mapping, > unsigned long lookahead_size) > { > struct inode *inode = mapping->host; > - struct page *page; > - unsigned long end_index; /* The last page we want to read */ > LIST_HEAD(page_pool); > loff_t isize = i_size_read(inode); > gfp_t gfp_mask = readahead_gfp_mask(mapping); > @@ -178,22 +176,29 @@ void __do_page_cache_readahead(struct address_space *mapping, > ._index = index, > }; > unsigned long i; > + pgoff_t end_index; /* The last page we want to read */ > > if (isize == 0) > return; > > - end_index = ((isize - 1) >> PAGE_SHIFT); > + end_index = (isize - 1) >> PAGE_SHIFT; > + if (index > end_index) > + return; > + /* 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; /* 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. - Eric