From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756471Ab1E1UU2 (ORCPT ); Sat, 28 May 2011 16:20:28 -0400 Received: from smtp-out.google.com ([74.125.121.67]:50770 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752212Ab1E1UU0 (ORCPT ); Sat, 28 May 2011 16:20:26 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=date:from:x-x-sender:to:cc:subject:message-id:user-agent :mime-version:content-type; b=fbRpx8p3UmROFZP1Ip2sicDmhODaR4ETJOmVoZ4Pss3baYhTquGPEZxM9wsQ6jd0YF jclBZEORRAE64O4zES/Q== Date: Sat, 28 May 2011 13:20:21 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@sister.anvils To: Linus Torvalds cc: Andrew Morton , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH] mm: fix page_lock_anon_vma leaving mutex locked Message-ID: User-Agent: Alpine 2.00 (LSU 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On one machine I've been getting hangs, a page fault's anon_vma_prepare() waiting in anon_vma_lock(), other processes waiting for that page's lock. This is a replay of last year's f18194275c39 "mm: fix hang on anon_vma->root->lock". The new page_lock_anon_vma() places too much faith in its refcount: when it has acquired the mutex_trylock(), it's possible that a racing task in anon_vma_alloc() has just reallocated the struct anon_vma, set refcount to 1, and is about to reset its anon_vma->root. Fix this by saving anon_vma->root, and relying on the usual page_mapped() check instead of a refcount check: if page is still mapped, the anon_vma is still ours; if page is not still mapped, we're no longer interested. Signed-off-by: Hugh Dickins --- mm/rmap.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) --- linux.orig/mm/rmap.c 2011-05-27 20:07:44.000000000 -0700 +++ linux/mm/rmap.c 2011-05-27 20:31:04.596303434 -0700 @@ -405,6 +405,7 @@ out: struct anon_vma *page_lock_anon_vma(struct page *page) { struct anon_vma *anon_vma = NULL; + struct anon_vma *root_anon_vma; unsigned long anon_mapping; rcu_read_lock(); @@ -415,13 +416,15 @@ struct anon_vma *page_lock_anon_vma(stru goto out; anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); - if (mutex_trylock(&anon_vma->root->mutex)) { + root_anon_vma = ACCESS_ONCE(anon_vma->root); + if (mutex_trylock(&root_anon_vma->mutex)) { /* - * If we observe a !0 refcount, then holding the lock ensures - * the anon_vma will not go away, see __put_anon_vma(). + * If the page is still mapped, then this anon_vma is still + * its anon_vma, and holding the mutex ensures that it will + * not go away, see __put_anon_vma(). */ - if (!atomic_read(&anon_vma->refcount)) { - anon_vma_unlock(anon_vma); + if (!page_mapped(page)) { + mutex_unlock(&root_anon_vma->mutex); anon_vma = NULL; } goto out;