From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D240CEE49B1 for ; Tue, 22 Aug 2023 18:42:45 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qYWKC-0004Af-BX; Tue, 22 Aug 2023 14:41:56 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qYWK0-00044w-2W for qemu-devel@nongnu.org; Tue, 22 Aug 2023 14:41:44 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qYWJy-00085z-0a for qemu-devel@nongnu.org; Tue, 22 Aug 2023 14:41:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1692729701; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=/3PZRAQFytwLbvAuiDekJz9xgmkxXQESXZOOnb1A+hY=; b=Ma/cooP8e6d5IwhSBv9E+/0iHevi8Sq/SsYLTWk0NTGQi/98lo9T1mr/rde0W6q1qJlk+Z dsAzpppf4Uaf0NVaxAiQouNm8eiy1QX1h1wZyJUJyBSbhCwND9DCVdXYKxomTe5fPX10Pa DPlT8TpsbTkW/WcyMBcHwupeA5y86FQ= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-562-ahjMq9H9M6Sz2fbKrVKGlw-1; Tue, 22 Aug 2023 14:41:36 -0400 X-MC-Unique: ahjMq9H9M6Sz2fbKrVKGlw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 84723101A59D; Tue, 22 Aug 2023 18:41:36 +0000 (UTC) Received: from localhost (unknown [10.39.195.69]) by smtp.corp.redhat.com (Postfix) with ESMTP id E1EDF64661; Tue, 22 Aug 2023 18:41:35 +0000 (UTC) Date: Tue, 22 Aug 2023 14:41:34 -0400 From: Stefan Hajnoczi To: Kevin Wolf Cc: qemu-block@nongnu.org, eblake@redhat.com, eesposit@redhat.com, pbonzini@redhat.com, vsementsov@yandex-team.ru, qemu-devel@nongnu.org Subject: Re: [PATCH 03/21] preallocate: Don't poll during permission updates Message-ID: <20230822184134.GF727224@fedora> References: <20230817125020.208339-1-kwolf@redhat.com> <20230817125020.208339-4-kwolf@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="zbWJqbEty2mkFLlU" Content-Disposition: inline In-Reply-To: <20230817125020.208339-4-kwolf@redhat.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org --zbWJqbEty2mkFLlU Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Aug 17, 2023 at 02:50:02PM +0200, Kevin Wolf wrote: > When the permission related BlockDriver callbacks are called, we are in > the middle of an operation traversing the block graph. Polling in such a > place is a very bad idea because the graph could change in unexpected > ways. In the future, callers will also hold the graph lock, which is > likely to turn polling into a deadlock. >=20 > So we need to get rid of calls to functions like bdrv_getlength() or > bdrv_truncate() there as these functions poll internally. They are > currently used so that when no parent has write/resize permissions on > the image any more, the preallocate filter drops the extra preallocated > area in the image file and gives up write/resize permissions itself. >=20 > In order to achieve this without polling in .bdrv_check_perm, don't > immediately truncate the image, but only schedule a BH to do so. The > filter keeps the write/resize permissions a bit longer now until the BH > has executed. >=20 > There is one case in which delaying doesn't work: Reopening the image > read-only. In this case, bs->file will likely be reopened read-only, > too, so keeping write permissions a bit longer on it doesn't work. But > we can already cover this case in preallocate_reopen_prepare() and not > rely on the permission updates for it. >=20 > Signed-off-by: Kevin Wolf > --- > block/preallocate.c | 89 +++++++++++++++++++++++++++++++++++---------- > 1 file changed, 69 insertions(+), 20 deletions(-) I don't know the permissions code well enough, but the patch matches the commit description: Acked-by: Stefan Hajnoczi --zbWJqbEty2mkFLlU Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmTlAV4ACgkQnKSrs4Gr c8iPcQgAkf200RL8oZXFI+m+JLoOeKlrvhlOgMOgHu8aUyYArqboPFziEPDBPeE/ kkyKvhCcQGnEjKSk2nOfYeWYGyo0AFscMturvr9kcnlU1ftwPT8xl/ruaXAYTCUU fqXbANPA8TvnAZ5BKMKJh4VGmKK49WsaL2xUF9ncslEQMkjCLzNcccOFcb8XwZg1 Ki3Pomp5plHb04MwyQZu/gyekWo2Ob8AQCaFf1S7M6jUHL3BFeThP0Ij0CMUv2KY aDxrlH6Ma/UJaSLJcZwhJiCk48+LA0fHZIKnVqz9BJKFVRCuYg4mi/7ZIYQrnFrT vfvlJSeXwMRbnAsJyfDJ1iqru6QP0w== =deuM -----END PGP SIGNATURE----- --zbWJqbEty2mkFLlU--