From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755479Ab1AKLpU (ORCPT ); Tue, 11 Jan 2011 06:45:20 -0500 Received: from gir.skynet.ie ([193.1.99.77]:33600 "EHLO gir.skynet.ie" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753534Ab1AKLpR (ORCPT ); Tue, 11 Jan 2011 06:45:17 -0500 Date: Tue, 11 Jan 2011 11:44:54 +0000 From: Mel Gorman To: Hugh Dickins Cc: Andrew Morton , Rik van Riel , Naoya Horiguchi , "Jun'ichi Nomura" , Andi Kleen , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: fix migration hangs on anon_vma lock Message-ID: <20110111114453.GC11932@csn.ul.ie> References: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17+20080114 (2008-01-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 10, 2011 at 11:08:04PM -0800, Hugh Dickins wrote: > Increased usage of page migration in mmotm reveals that the anon_vma > locking in unmap_and_move() has been deficient since 2.6.36 (or even > earlier). Hmm, a certain amount of the compaction work was spent fixing migration bugs. I wonder if there are mysterious bug reports out there related to the use of move_pages() that are only getting fixed now. > Review at the time of f18194275c39835cb84563500995e0d503a32d9a > "mm: fix hang on anon_vma->root->lock" missed the issue here: the anon_vma > to which we get a reference may already have been freed back to its slab > (it is in use when we check page_mapped, but that can change), and so its > anon_vma->root may be switched at any moment by reuse in anon_vma_prepare. > > Perhaps we could fix that with a get_anon_vma_unless_zero(), but let's not: > just rely on page_lock_anon_vma() to do all the hard thinking for us, then > we don't need any rcu read locking over here. > > In removing the rcu_unlock label: since PageAnon is a bit in page->mapping, > it's impossible for a !page->mapping page to be anon; but insert VM_BUG_ON > in case the implementation ever changes. > > Signed-off-by: Hugh Dickins > Cc: stable@kernel.org [2.6.37, 2.6.36] Reasoning and patch look correct. Light testing did not show up any obvious problems. Reviewed-by: Mel Gorman -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab