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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D66F1C7EE25 for ; Wed, 7 Jun 2023 20:46:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234858AbjFGUqy (ORCPT ); Wed, 7 Jun 2023 16:46:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46462 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234810AbjFGUqV (ORCPT ); Wed, 7 Jun 2023 16:46:21 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1509F2109; Wed, 7 Jun 2023 13:46:12 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id C4D1D21A08; Wed, 7 Jun 2023 20:46:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1686170770; h=from:from:reply-to: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=2X/4z9UYBzSMTV7e6BXpQiPPRmpVPkn6HQVqfRrAQo4=; b=It9MFa8lj8NbG+tYYqcdLFdgMcpVrEq1LzUE2UIXnHDDMbVAOJFiQFPM8Jhjt/2+oP5obE AoTbfz8sqSJjM74m+rStcDG5GUgASl5X/ii0oFzt8f0Kvt548rTHQNVJVeJNP2qTx/g/hh EdaWwKUKmGHhVGJE7/h/UrzyxJw1sDU= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1686170770; h=from:from:reply-to: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=2X/4z9UYBzSMTV7e6BXpQiPPRmpVPkn6HQVqfRrAQo4=; b=l1IXYqHMoWJyykYWDl6Nb470g/LXhVrfw0k+jGnpjT70TqgKDizeBeMzkj92X5SLHhWIgc JMIe/NYsBVAnUkBQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id B0D3A1346D; Wed, 7 Jun 2023 20:46:10 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id uEEhK5LsgGT2QQAAMHmgww (envelope-from ); Wed, 07 Jun 2023 20:46:10 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 3D9E5A0749; Wed, 7 Jun 2023 22:46:10 +0200 (CEST) Date: Wed, 7 Jun 2023 22:46:10 +0200 From: Jan Kara To: "Darrick J. Wong" Cc: Jan Kara , Luis Chamberlain , hch@infradead.org, sandeen@sandeen.net, song@kernel.org, rafael@kernel.org, gregkh@linuxfoundation.org, viro@zeniv.linux.org.uk, jikos@kernel.org, bvanassche@acm.org, ebiederm@xmission.com, mchehab@kernel.org, keescook@chromium.org, p.raghav@samsung.com, da.gomez@samsung.com, linux-fsdevel@vger.kernel.org, kernel@tuxforce.de, kexec@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 3/6] fs: distinguish between user initiated freeze and kernel initiated freeze Message-ID: <20230607204610.5ai5cleks6qzjal7@quack3> References: <20230508011717.4034511-1-mcgrof@kernel.org> <20230508011717.4034511-4-mcgrof@kernel.org> <20230522234200.GC11598@frogsfrogsfrogs> <20230525141430.slms7f2xkmesezy5@quack3> <20230607163110.GC72224@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230607163110.GC72224@frogsfrogsfrogs> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 07-06-23 09:31:10, Darrick J. Wong wrote: > On Thu, May 25, 2023 at 04:14:30PM +0200, Jan Kara wrote: > > On Mon 22-05-23 16:42:00, Darrick J. Wong wrote: > > > How about this as an alternative patch? Kernel and userspace freeze > > > state are stored in s_writers; each type cannot block the other (though > > > you still can't have nested kernel or userspace freezes); and the freeze > > > is maintained until /both/ freeze types are dropped. > > > > > > AFAICT this should work for the two other usecases (quiescing pagefaults > > > for fsdax pmem pre-removal; and freezing fses during suspend) besides > > > online fsck for xfs. > > > > > > --D > > > > > > From: Darrick J. Wong > > > Subject: fs: distinguish between user initiated freeze and kernel initiated freeze > > > > > > Userspace can freeze a filesystem using the FIFREEZE ioctl or by > > > suspending the block device; this state persists until userspace thaws > > > the filesystem with the FITHAW ioctl or resuming the block device. > > > Since commit 18e9e5104fcd ("Introduce freeze_super and thaw_super for > > > the fsfreeze ioctl") we only allow the first freeze command to succeed. > > > > > > The kernel may decide that it is necessary to freeze a filesystem for > > > its own internal purposes, such as suspends in progress, filesystem fsck > > > activities, or quiescing a device prior to removal. Userspace thaw > > > commands must never break a kernel freeze, and kernel thaw commands > > > shouldn't undo userspace's freeze command. > > > > > > Introduce a couple of freeze holder flags and wire it into the > > > sb_writers state. One kernel and one userspace freeze are allowed to > > > coexist at the same time; the filesystem will not thaw until both are > > > lifted. > > > > > > Inspired-by: Luis Chamberlain > > > Signed-off-by: Darrick J. Wong > > > > Yes, this is exactly how I'd imagine it. Thanks for writing the patch! > > > > I'd just note that this would need rebasing on top of Luis' patches 1 and > > 2. Also: > > > > > + if (sbw->frozen == SB_FREEZE_COMPLETE) { > > > + switch (who) { > > > + case FREEZE_HOLDER_KERNEL: > > > + if (sbw->freeze_holders & FREEZE_HOLDER_KERNEL) { > > > + /* > > > + * Kernel freeze already in effect; caller can > > > + * try again. > > > + */ > > > + deactivate_locked_super(sb); > > > + return -EBUSY; > > > + } > > > + if (sbw->freeze_holders & FREEZE_HOLDER_USERSPACE) { > > > + /* > > > + * Share the freeze state with the userspace > > > + * freeze already in effect. > > > + */ > > > + sbw->freeze_holders |= who; > > > + deactivate_locked_super(sb); > > > + return 0; > > > + } > > > + break; > > > + case FREEZE_HOLDER_USERSPACE: > > > + if (sbw->freeze_holders & FREEZE_HOLDER_USERSPACE) { > > > + /* > > > + * Userspace freeze already in effect; tell > > > + * the caller we're busy. > > > + */ > > > + deactivate_locked_super(sb); > > > + return -EBUSY; > > > + } > > > + if (sbw->freeze_holders & FREEZE_HOLDER_KERNEL) { > > > + /* > > > + * Share the freeze state with the kernel > > > + * freeze already in effect. > > > + */ > > > + sbw->freeze_holders |= who; > > > + deactivate_locked_super(sb); > > > + return 0; > > > + } > > > + break; > > > + default: > > > + BUG(); > > > + deactivate_locked_super(sb); > > > + return -EINVAL; > > > + } > > > + } > > > > Can't this be simplified to: > > > > BUG_ON(who & ~(FREEZE_HOLDER_USERSPACE | FREEZE_HOLDER_KERNEL)); > > BUG_ON(!(!(who & FREEZE_HOLDER_USERSPACE) ^ > > !(who & FREEZE_HOLDER_KERNEL))); > > retry: > > if (sb->s_writers.freeze_holders & who) > > return -EBUSY; > > /* Already frozen by someone else? */ > > if (sb->s_writers.freeze_holders & ~who) { > > sb->s_writers.freeze_holders |= who; > > return 0; > > } > > > > Now the only remaining issue with the code is that the two different > > holders can be attempting to freeze the filesystem at once and in that case > > one of them has to wait for the other one instead of returning -EBUSY as > > would happen currently. This can happen because we temporarily drop > > s_umount in freeze_super() due to lock ordering issues. I think we could > > do something like: > > > > if (!sb_unfrozen(sb)) { > > up_write(&sb->s_umount); > > wait_var_event(&sb->s_writers.frozen, > > sb_unfrozen(sb) || sb_frozen(sb)); > > down_write(&sb->s_umount); > > goto retry; > > } > > > > and then sprinkle wake_up_var(&sb->s_writers.frozen) at appropriate places > > in freeze_super(). > > If we implemented this behavior change, it ought to be a separate patch. > > For the case where the kernel is freezing the fs and userspace wants to > start freezing the fs, we could make userspace wait and then share the > kernel freeze. Yes. > For any case where the fs is !unfrozen and the kernel wants to start > freezing the fs, I think I'd rather return EBUSY immediately and let the > caller decide to wait and/or call back. Possibly, although I thought that if userspace has frozen the fs and kernel wants to freeze, we want to return success? At least that was what I think your patches were doing. And then I don't see the point why we should be returning EBUSY if userspace is in the middle of the freeze. So what's the intended semantics? > For the case where one userspace thread is freezing the fs and another > userspace thread wants to start freezing the fs, I think the current > behavior of returning EBUSY immediately is ok. Yes. Honza -- Jan Kara SUSE Labs, CR