From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx144.postini.com [74.125.245.144]) by kanga.kvack.org (Postfix) with SMTP id 30A9B6B0069 for ; Tue, 18 Sep 2012 18:09:36 -0400 (EDT) Date: Tue, 18 Sep 2012 15:09:33 -0700 From: Andrew Morton Subject: Re: [PATCH v10 1/5] mm: introduce a common interface for balloon pages mobility Message-Id: <20120918150933.cab895b8.akpm@linux-foundation.org> In-Reply-To: <20120918162420.GB1645@optiplex.redhat.com> References: <89c9f4096bbad072e155445fcdf1805d47ddf48e.1347897793.git.aquini@redhat.com> <20120917151543.fd523040.akpm@linux-foundation.org> <20120918162420.GB1645@optiplex.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Rafael Aquini Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Rusty Russell , "Michael S. Tsirkin" , Rik van Riel , Mel Gorman , Andi Kleen , Konrad Rzeszutek Wilk , Minchan Kim , Peter Zijlstra , "Paul E. McKenney" On Tue, 18 Sep 2012 13:24:21 -0300 Rafael Aquini wrote: > On Mon, Sep 17, 2012 at 03:15:43PM -0700, Andrew Morton wrote: > > > +/* return code to identify when a ballooned page has been migrated */ > > > +#define BALLOON_MIGRATION_RETURN 0xba1100 > > > > I didn't really spend enough time to work out why this was done this > > way, but I know a hack when I see one! > > > Yes, I'm afraid it's a hack, but, unfortunately, it's a necessary one (IMHO). > > This 'distinct' return code is used to flag a sucessful balloon page migration > at the following unmap_and_move() snippet (patch 2). > If by any reason we fail to identify a sucessfull balloon page migration, we > will cause a page leak, as the old 'page' won't be properly released. > ..... > rc = __unmap_and_move(page, newpage, force, offlining, mode); > + > + if (unlikely(rc == BALLOON_MIGRATION_RETURN)) { > + /* > + * A ballooned page has been migrated already. > + * Now, it's the time to remove the old page from the isolated > + * pageset list and handle it back to Buddy, wrap-up counters > + * and return. > + */ > ...... > > By reaching that point in code, we cannot rely on testing page->mapping flags > anymore for both 'page' and 'newpage' because: > a) migration has already finished and 'page'->mapping is wiped out; > b) balloon might have started to deflate, and 'newpage' might be released > already; > > If the return code approach is unnaceptable, we might defer the 'page'->mapping > wipe-out step to that point in code for the balloon page case. > That, however, tends to be a little bit heavier, IMHO, as it will require us to > acquire the page lock once more to proceed the mapping wipe out, thus > potentially introducing overhead by lock contention (specially when several > parallel compaction threads are scanning pages for isolation) I think the return code approach _is_ acceptable, but the implementation could be improved. As it stands, a naive caller could be expecting either 0 (success) or a negative errno. A large positive return value could trigger havoc. We can defend against such programming mistakes with code commentary, but a better approach would be to enumerate the return values. Along the lines of /* * Return values from addresss_space_operations.migratepage(). Returns a * negative errno on failure. */ #define MIGRATEPAGE_SUCCESS 0 #define MIGRATEPAGE_BALLOON_THINGY 1 /* nice comment goes here */ and convert all callers to explicitly check for MIGRATEPAGE_SUCCESS, not literal zero. We should be particularly careful to look for codesites which are unprepared for positive return values, such as ret = migratepage(); if (ret < 0) return ret; ... return ret; /* success!! */ If we wanted to be really vigilant about this, we could do #define MIGRATEPAGE_SUCCESS 1 #define MIGRATEPAGE_BALLOON_THINGY 2 so any naive code which tests for literal zero will nicely explode early in testing. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org