From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: [PATCH 02/12] nilfs2: use setup_bdev_super to de-duplicate the mount code Date: Thu, 10 Aug 2023 13:05:47 +0200 Message-ID: <20230810110547.ks62g2flysgwpgru@quack3> References: <20230802154131.2221419-1-hch@lst.de> <20230802154131.2221419-3-hch@lst.de> <20230803114651.ihtqqgthbdjjgxev@quack3> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1691665548; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=1DFh8evqtxspjStydxXxKkjnilpW0tYumnluwrTMIZo=; b=vLT7nxhsKaVB+2d6j1Yf5X8t1GLYqRuuN4g6ZutMMICOunL7y0FZNv7gLZiYXzC/l2Xomu I8xmm0CPUZCp/iFMbz+eWr+/L/YJb1/utRAjSeiWYdJX1v/2EQl6s2syK6Yg4TzJfLlWXK j+SruuD6SzbR8Z3B6IZKMOfh62510CU= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1691665548; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=1DFh8evqtxspjStydxXxKkjnilpW0tYumnluwrTMIZo=; b=gjop8gtt3TnFXcbkSm06MHlxNzKkmd+CJQt27qbqqSRylA/dQexstbBZ/MQ0KnNaiV041a Mrb6ToOuPtQHkpAw== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="windows-1252" To: Ryusuke Konishi Cc: Jan Kara , Christoph Hellwig , Al Viro , Christian Brauner , Chris Mason , Josef Bacik , David Sterba , Theodore Ts'o , Andreas Dilger , Jaegeuk Kim , Chao Yu , "Darrick J. Wong" , Jens Axboe , linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-block@vger.kernel.org On Fri 04-08-23 11:01:39, Ryusuke Konishi wrote: > On Thu, Aug 3, 2023 at 8:46=E2=80=AFPM Jan Kara wrote: > > > > On Wed 02-08-23 17:41:21, Christoph Hellwig wrote: > > > Use the generic setup_bdev_super helper to open the main block device > > > and do various bits of superblock setup instead of duplicating the > > > logic. This includes moving to the new scheme implemented in common > > > code that only opens the block device after the superblock has alloca= ted. > > > > > > It does not yet convert nilfs2 to the new mount API, but doing so will > > > become a bit simpler after this first step. > > > > > > Signed-off-by: Christoph Hellwig > > > > AFAICS nilfs2 could *almost* use mount_bdev() directly and then just do= its >=20 > > snapshot thing after mount_bdev() returns. But it has this weird logic > > that: "if the superblock is already mounted but we can shrink the whole > > dcache, then do remount instead of ignoring mount options". Firstly, th= is > > looks racy - what prevents someone from say opening a file on the sb ju= st > > after nilfs_tree_is_busy() shrinks dcache? Secondly, it is inconsistent > > with any other filesystem so it's going to surprise sysadmins not > > intimately knowing nilfs2. Thirdly, from userspace you cannot tell what > > your mount call is going to do. Last but not least, what is it really g= ood > > for? Ryusuke, can you explain please? > > > > Honza >=20 > I think you are referring to the following part: >=20 > > if (!s->s_root) { > ... > > } else if (!sd.cno) { > > if (nilfs_tree_is_busy(s->s_root)) { > > if ((flags ^ s->s_flags) & SB_RDONLY) { > > nilfs_err(s, > > "the device already has a %s m= ount.", > > sb_rdonly(s) ? "read-only" : "= read/write"); > > err =3D -EBUSY; > > goto failed_super; > > } > > } else { > > /* > > * Try remount to setup mount states if the curr= ent > > * tree is not mounted and only snapshots use th= is sb. > > */ > > err =3D nilfs_remount(s, &flags, data); > > if (err) > > goto failed_super; > > } > > } >=20 > What this logic is trying to do is, if there is already a nilfs2 mount > instance for the device, and are trying to mounting the current tree > (sd.cno is 0, so this is not a snapshot mount), then will switch > depending on whether the current tree has a mount: >=20 > - If the current tree is mounted, it's just like a normal filesystem. > (A read-only mount and a read/write mount can't coexist, so check > that, and reuse the instance if possible) > - Otherwise, i.e. for snapshot mounts only, do whatever is necessary > to add a new current mount, such as starting a log writer. > Since it does the same thing that nilfs_remount does, so > nilfs_remount() is used there. >=20 > Whether or not there is a current tree mount can be determined by > d_count(s->s_root) > 1 as nilfs_tree_is_busy() does. > Where s->s_root is always the root dentry of the current tree, not > that of the mounted snapshot. I see now, thanks for explanation! But one thing still is not clear to me. If you say have a snapshot mounted read-write and then you mount the current snapshot (cno =3D=3D 0) read-only, you'll switch the whole superblo= ck to read-only state. So also the mounted snapshot is suddently read-only which is unexpected and actually supposedly breaks things because you can still have file handles open for writing on the snapshot etc.. So how do you solve that? Honza --=20 Jan Kara SUSE Labs, CR