Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964800AbZJOXt3 (ORCPT ); Thu, 15 Oct 2009 19:49:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S935626AbZJOXt3 (ORCPT ); Thu, 15 Oct 2009 19:49:29 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:59044 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935608AbZJOXt2 (ORCPT ); Thu, 15 Oct 2009 19:49:28 -0400 X-SecurityPolicyCheck-FJ: OK by FujitsuOutboundMailChecker v1.3.1 Date: Fri, 16 Oct 2009 08:46:21 +0900 From: KAMEZAWA Hiroyuki To: Hugh Dickins Cc: Andrew Morton , Nigel Cunningham , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 2/9] swap_info: change to array of pointers Message-Id: <20091016084621.7199411d.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: References: <20091015111107.b505b676.kamezawa.hiroyu@jp.fujitsu.com> Organization: FUJITSU Co. LTD. X-Mailer: Sylpheed 2.5.0 (GTK+ 2.10.14; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4642 Lines: 140 On Thu, 15 Oct 2009 23:41:19 +0100 (BST) Hugh Dickins wrote: > On Thu, 15 Oct 2009, KAMEZAWA Hiroyuki wrote: > > On Thu, 15 Oct 2009 01:48:01 +0100 (BST) > > Hugh Dickins wrote: > > > --- si1/mm/swapfile.c 2009-10-14 21:25:58.000000000 +0100 > > > +++ si2/mm/swapfile.c 2009-10-14 21:26:09.000000000 +0100 > > > @@ -49,7 +49,7 @@ static const char Unused_offset[] = "Unu > > > > > > static struct swap_list_t swap_list = {-1, -1}; > > > > > > -static struct swap_info_struct swap_info[MAX_SWAPFILES]; > > > +static struct swap_info_struct *swap_info[MAX_SWAPFILES]; > > > > > > > Could you add some comment like this ? > > == > > nr_swapfile is never decreased. > > swap_info[type] pointer will never be invalid if it turns to be valid once. > > > > > > for (i = 0; i < nr_swapfiles; i++) { > > smp_rmp(); > > sis = swap_info[type]; > > .... > > } > > Then, we can execute above without checking sis is valid or not. > > smp_rmb() is required when we do above loop without swap_lock(). > > I do describe this (too briefly?) in the comment on smp_wmb() where > swap_info[type] is set and nr_swapfiles raised, in swapon (see below). > And make a quick same-line comment on the corresponding smp_rmb()s. > > Those seem more useful to me than such a comment on the > static struct swap_info_struct *swap_info[MAX_SWAPFILES]; > yes. > I was about to add (now, in writing this mail) that /proc/swaps is > the only thing that reads them without swap_lock; but that's not > true, of course, swap_duplicate and swap_free (or their helpers) > make preliminary checks without swap_lock - but the difference > there is that (unless the pagetable has become corrupted) they're > dealing with a swap entry which was previously valid, so can by > this time rely upon swap_info[type] and nr_swapfiles to be safe. > Ah, a fact which confusing me very much at frist look was - nr_swapfiles nerver decreases - then, swap_info[type] will be never freed once allocated I hope this should be commented upon this. static struct swap_info_struct *swap_info[MAX_SWAPFILES]; Regards, -Kame > > swapon_mutex() will be no help. > > > > Whether sis is used or not can be detelcted by sis->flags. > > > > > @@ -1675,11 +1674,13 @@ static void *swap_start(struct seq_file > > > if (!l) > > > return SEQ_START_TOKEN; > > > > > > - for (i = 0; i < nr_swapfiles; i++, ptr++) { > > > - if (!(ptr->flags & SWP_USED) || !ptr->swap_map) > > > + for (type = 0; type < nr_swapfiles; type++) { > > > + smp_rmb(); /* read nr_swapfiles before swap_info[type] */ > > > + si = swap_info[type]; > > > > if (!si) ? > > > > > + if (!(si->flags & SWP_USED) || !si->swap_map) > > > continue; > > > if (!--l) > > > - return ptr; > > > + return si; > > > } > ... > > > static void *swap_next(struct seq_file *swap, void *v, loff_t *pos) > > > { > > > - struct swap_info_struct *ptr; > > > - struct swap_info_struct *endptr = swap_info + nr_swapfiles; > > > + struct swap_info_struct *si = v; > > > + int type; > > > > > > if (v == SEQ_START_TOKEN) > > > - ptr = swap_info; > > > - else { > > > - ptr = v; > > > - ptr++; > > > - } > > > + type = 0; > > > + else > > > + type = si->type + 1; > > > > > > - for (; ptr < endptr; ptr++) { > > > - if (!(ptr->flags & SWP_USED) || !ptr->swap_map) > > > + for (; type < nr_swapfiles; type++) { > > > + smp_rmb(); /* read nr_swapfiles before swap_info[type] */ > > > + si = swap_info[type]; > > > + if (!(si->flags & SWP_USED) || !si->swap_map) > ... > > > @@ -1799,23 +1800,45 @@ SYSCALL_DEFINE2(swapon, const char __use > ... > > > - if (type >= nr_swapfiles) > > > - nr_swapfiles = type+1; > > > - memset(p, 0, sizeof(*p)); > > > INIT_LIST_HEAD(&p->extent_list); > > > + if (type >= nr_swapfiles) { > > > + p->type = type; > > > + swap_info[type] = p; > > > + /* > > > + * Write swap_info[type] before nr_swapfiles, in case a > > > + * racing procfs swap_start() or swap_next() is reading them. > > > + * (We never shrink nr_swapfiles, we never free this entry.) > > > + */ > > > + smp_wmb(); > > > + nr_swapfiles++; > > > + } else { > > > + kfree(p); > > > + p = swap_info[type]; > > > + /* > > > + * Do not memset this entry: a racing procfs swap_next() > > > + * would be relying on p->type to remain valid. > > > + */ > > > + } > ... > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/