From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760289AbYFSQBU (ORCPT ); Thu, 19 Jun 2008 12:01:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754118AbYFSQBF (ORCPT ); Thu, 19 Jun 2008 12:01:05 -0400 Received: from vpn.id2.novell.com ([195.33.99.129]:35058 "EHLO vpn.id2.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754021AbYFSQBE convert rfc822-to-8bit (ORCPT ); Thu, 19 Jun 2008 12:01:04 -0400 Message-Id: <485A9EF5.76E4.0078.0@novell.com> X-Mailer: Novell GroupWise Internet Agent 7.0.3 Date: Thu, 19 Jun 2008 17:01:25 +0100 From: "Jan Beulich" To: "Jeremy Fitzhardinge" Cc: , , , Subject: Re: [PATCH] i386: fix vmalloc_sync_all() for Xen References: <48591049.76E4.0078.0@novell.com> <485969A4.6090406@goop.org> <485A465D.76E4.0078.0@novell.com> <485A7122.8090200@goop.org> In-Reply-To: <485A7122.8090200@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>>> + if (!test_bit(sync_index(address), insync)) { >>>> >>>> >>> It's probably worth reversing this test and removing a layer of indentation. >>> >> >> How? There's a second if() following this one, so we can't just 'continue;' >> here. >> > >That second if() block seems completely redundant: > > if (address == start && test_bit(pgd_index(address), insync)) > start = address + PGDIR_SIZE; > >All it does it update "start", but start isn't used anywhere else in the >loop. Since start is a static variable, it must be updated this way. The intention here is to shorten the loop in later runs - since kernel page table entries never go away, this is possible. Possibly just using the insync array would be sufficient, but when I first coded this I wanted to avoid as much overhead as was possible. >>>> spin_lock_irqsave(&pgd_lock, flags); >>>> + if (unlikely(list_empty(&pgd_list))) { >>>> + spin_unlock_irqrestore(&pgd_lock, flags); >>>> + return; >>>> + } >>>> >>>> >>> This seems a bit warty. If the list is empty, then won't the >>> list_for_each_entry() just fall through? Presumably this only applies >>> to boot, since pgd_list won't be empty on a running system with usermode >>> processes. Is there a correctness issue here, or is it just a >>> micro-optimisation? >>> >> >> No, it isn't. Note the setting to NULL of page, which after the loop gets >> tested for. list_for_each_entry() would never yield a NULL page, even >> if the list is empty. > >Does that matter? If pgd_list is empty, then it's in sync by >definition. Why does it need special-casing? Yes, certainly. But it would result in all insync bits set, which would be wrong - only non-empty page directory entries can be in sync. >>>> list_for_each_entry(page, &pgd_list, lru) { >>>> if (!vmalloc_sync_one(page_address(page), >>>> - address)) >>>> + address)) { >>>> + BUG_ON(list_first_entry(&pgd_list, >>>> + struct page, >>>> + lru) != page); >>>> >>>> >>> What condition is this testing for? >>> >> >> This is a replacement of the BUG_ON() that an earlier patch from you >> removed: Failure of vmalloc_sync_one() must happen on the first >> entry or never, and this is what is being checked for here. >> > >Could you add a comment? Sure, though there was none originally, and the intention seemed quite clear to me. Jan