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 C6328C2BA83 for ; Wed, 12 Feb 2020 17:52:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9169F21739 for ; Wed, 12 Feb 2020 17:52:51 +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="G4041bdk" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728897AbgBLRwv (ORCPT ); Wed, 12 Feb 2020 12:52:51 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:60560 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727231AbgBLRwu (ORCPT ); Wed, 12 Feb 2020 12:52:50 -0500 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=PC1JYyytBxzYVOWjA8HahlODsHQReOcYQHnl2qvSG/I=; b=G4041bdkHZ6mUZsGWvbHmafyzc d8eKGbikKMu1/Xs3Vqe19YzLl3Bc4o+/WsXU/5dPCcWBWTq57avYyMJBVo7HDek6/QMWnfeCw+suV PA1t+NZ9QDXVj+3SNEGLQRMv3wUIj2goGXRQeN7YL3IqICljs1egL/JFlg78R3bHPmltupujDXOOV mg44/ZCaRx+qFQTT/WuFlZ3yrT8O/GggnQHIT0wSGf0fgckbd8hsibPPqbi2opqVz3mwlktH2ZY5+ a5QIEnYFgYTnaZ4AoG0SXjFcKQADdnZqPgRWpOazSvbj0J8M9WKPqR8Qk4EcYmGg+oUDbKoxOnOJ2 cIuAvtDQ==; Received: from hch by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j1wBu-0003U1-93; Wed, 12 Feb 2020 17:52:50 +0000 Date: Wed, 12 Feb 2020 09:52:50 -0800 From: Christoph Hellwig To: Matthew Wilcox Cc: Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 02/25] mm: Optimise find_subpage for !THP Message-ID: <20200212175250.GA11424@infradead.org> References: <20200212041845.25879-1-willy@infradead.org> <20200212041845.25879-3-willy@infradead.org> <20200212074105.GE7068@infradead.org> <20200212130200.GC7778@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200212130200.GC7778@bombadil.infradead.org> X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 12, 2020 at 05:02:00AM -0800, Matthew Wilcox wrote: > > Can you add comments describing the use case of this function and why > > it does all these checks? It looks like black magic to me. > > Would this help? > > -static inline struct page *find_subpage(struct page *page, pgoff_t offset) > +/* > + * Given the page we found in the page cache, return the page corresponding > + * to this offset in the file > + */ > +static inline struct page *find_subpage(struct page *head, pgoff_t offset) > { > - if (PageHuge(page)) > - return page; > + /* HugeTLBfs wants the head page regardless */ > + if (PageHuge(head)) > + return head; > > - VM_BUG_ON_PAGE(PageTail(page), page); > + VM_BUG_ON_PAGE(PageTail(head), head); > > - return page + (offset & (hpage_nr_pages(page) - 1)); > + return head + (offset & (hpage_nr_pages(head) - 1)); Much better.