From owner-freebsd-stable@FreeBSD.ORG Sun Nov 17 09:49:33 2013 Return-Path: Delivered-To: freebsd-stable@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 8C352E04; Sun, 17 Nov 2013 09:49:33 +0000 (UTC) Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140]) by mx1.freebsd.org (Postfix) with ESMTP id 5CDE42387; Sun, 17 Nov 2013 09:49:31 +0000 (UTC) Received: from porto.starpoint.kiev.ua (porto-e.starpoint.kiev.ua [212.40.38.100]) by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id LAA24551; Sun, 17 Nov 2013 11:49:30 +0200 (EET) (envelope-from avg@FreeBSD.org) Received: from localhost ([127.0.0.1]) by porto.starpoint.kiev.ua with esmtp (Exim 4.34 (FreeBSD)) id 1Vhyyv-000Bef-Mr; Sun, 17 Nov 2013 11:49:29 +0200 Message-ID: <52889105.7040404@FreeBSD.org> Date: Sun, 17 Nov 2013 11:48:53 +0200 From: Andriy Gapon User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Steven Hartland , hartzell@alerce.com, freebsd-stable@FreeBSD.org Subject: Re: Help with filing a [maybe] ZFS/mmap bug. References: <20967.760.95825.310085@gargle.gargle.HOWL><51E80B30.1090004@FreeBSD.org><20968.10645.880772.30501@gargle.gargle.HOWL><520202E5.30300@FreeBSD.org><20994.55913.93606.436124@gargle.gargle.HOWL> <21111.12085.958991.356982@gargle.gargle.HOWL> <4EB902F80CE84DD2BF36C85EF4CE8EF8@multiplay.co.uk> <5284B8A5.8040604@FreeBSD.org> In-Reply-To: <5284B8A5.8040604@FreeBSD.org> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.16 Precedence: list List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 17 Nov 2013 09:49:33 -0000 on 14/11/2013 13:48 Andriy Gapon said the following: > HOWEVER. I think that there is a bug that I introduced in r246293. > Specifically I changed > vm_page_undirty(pp); > to > pmap_remove_write(pp); > vm_page_clear_dirty(pp, off, nbytes); > > vm_page_undirty() would be a very serious (and probably obvious) bug, if it were > not a NOP in effect. The details are explained in the commit message. > But when I used vm_page_clear_dirty I completely missed the fact that *extends* > the range to DEV_BSIZE aligned boundaries[*]. So, given the described behavior > and that pmap_remove_write clears the page modified bit, it is possible that the > data dirty data in the extended areas will be marked as clean. Here is a patch (for head) that should fix the described above issue: diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c index 2e2cbd6..4fcd571 100644 --- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c +++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c @@ -328,6 +328,20 @@ page_busy(vnode_t *vp, int64_t start, int64_t off, int64_t nbytes) { vm_object_t obj; vm_page_t pp; + int64_t end; + + /* + * At present vm_page_clear_dirty extends the cleared range to DEV_BSIZE + * aligned boundaries, if the range is not aligned. As a result a + * DEV_BSIZE subrange with partially dirty data may get marked as clean. + * It may happen that all DEV_BSIZE subranges are marked clean and thus + * the whole page would be considred clean despite have some dirty data. + * For this reason we should shrink the range to DEV_BSIZE aligned + * boundaries before calling vm_page_clear_dirty. + */ + end = rounddown2(off + nbytes, DEV_BSIZE); + off = roundup2(off, DEV_BSIZE); + nbytes = end - off; obj = vp->v_object; zfs_vmobject_assert_wlocked(obj); @@ -362,7 +376,8 @@ page_busy(vnode_t *vp, int64_t start, int64_t off, int64_t nbytes) ASSERT3U(pp->valid, ==, VM_PAGE_BITS_ALL); vm_object_pip_add(obj, 1); pmap_remove_write(pp); - vm_page_clear_dirty(pp, off, nbytes); + if (nbytes != 0) + vm_page_clear_dirty(pp, off, nbytes); } break; } -- Andriy Gapon