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 X-Spam-Level: X-Spam-Status: No, score=-15.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0E2E0C433B4 for ; Mon, 17 May 2021 17:30:31 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 5C32660FDB for ; Mon, 17 May 2021 17:30:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5C32660FDB Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:60128 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lih4X-00035J-8S for qemu-devel@archiver.kernel.org; Mon, 17 May 2021 13:30:29 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:50392) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lih1A-0001Zl-9e for qemu-devel@nongnu.org; Mon, 17 May 2021 13:27:01 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:31234) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lih17-0002zr-JE for qemu-devel@nongnu.org; Mon, 17 May 2021 13:26:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1621272416; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=po8n2trLdN1niCg1EdgfOWWTeBmMt5rR3G9Snj/oSzw=; b=Fb+anZwVsmqhM4QIYfK8RdxQWpV2Tn3zSbfQbIY9gxw1ME2k1ew/SmzU0vGrKSS+QMXXkH Aykatcf1mkhfS//Rk9UcvwORD0Q14z8c/XHdFU6fXO+MxfODV+UoIjbxeShHc9+SpfLGbv DLpK6KKEQzZCSxARAu89/xCc/nazmfY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-362-UvVjjZGSPaWZXrvpvcX9Ow-1; Mon, 17 May 2021 13:26:41 -0400 X-MC-Unique: UvVjjZGSPaWZXrvpvcX9Ow-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 927C31854E21 for ; Mon, 17 May 2021 17:26:40 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-113-29.ams2.redhat.com [10.36.113.29]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1AECB2B3BB; Mon, 17 May 2021 17:26:30 +0000 (UTC) Subject: Re: [PATCH 1/3] virtiofsd: Find original inode ID of mount points To: Vivek Goyal References: <20210512125544.9536-1-mreitz@redhat.com> <20210512125544.9536-2-mreitz@redhat.com> <20210517145739.GE546943@horse.lan> From: Max Reitz Message-ID: <13641be2-e875-ca43-1cc4-8d06a4e6f81c@redhat.com> Date: Mon, 17 May 2021 19:26:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210517145739.GE546943@horse.lan> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.374, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs-list , qemu-devel@nongnu.org, Stefan Hajnoczi , "Dr . David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 17.05.21 16:57, Vivek Goyal wrote: > On Wed, May 12, 2021 at 02:55:42PM +0200, Max Reitz wrote: >> Mount point directories represent two inodes: On one hand, they are a >> normal directory on their parent filesystem. On the other, they are the >> root node of the filesystem mounted there. Thus, they have two inode >> IDs. >> >> Right now, we only report the latter inode ID (i.e. the inode ID of the >> mounted filesystem's root node). This is fine once the guest has >> auto-mounted a submount there (so this inode ID goes with a device ID >> that is distinct from the parent filesystem), but before the auto-mount, >> they have the device ID of the parent and the inode ID for the submount. >> This is problematic because this is likely exactly the same >> st_dev/st_ino combination as the parent filesystem's root node. This >> leads to problems for example with `find`, which will thus complain >> about a filesystem loop if it has visited the parent filesystem's root >> node before, and then refuse to descend into the submount. >> >> There is a way to find the mount directory's original inode ID, and that >> is to readdir(3) the parent directory, look for the mount directory, and >> read the dirent.d_ino field. Using this, we can let lookup and >> readdirplus return that original inode ID, which the guest will thus >> show until the submount is auto-mounted. (Then, it will invoke getattr >> and that stat(2) call will return the inode ID for the submount.) >> >> Signed-off-by: Max Reitz >> --- >> tools/virtiofsd/passthrough_ll.c | 104 +++++++++++++++++++++++++++++-- >> 1 file changed, 99 insertions(+), 5 deletions(-) >> >> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c >> index 1553d2ef45..110b6e7e5b 100644 >> --- a/tools/virtiofsd/passthrough_ll.c >> +++ b/tools/virtiofsd/passthrough_ll.c >> @@ -968,14 +968,87 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname, >> return 0; >> } >> >> +/* >> + * Use readdir() to find mp_name's inode ID on the parent's filesystem. >> + * (For mount points, stat() will only return the inode ID on the >> + * filesystem mounted there, i.e. the root directory's inode ID. The >> + * mount point originally was a directory on the parent filesystem, >> + * though, and so has a different inode ID there. When passing >> + * submount information to the guest, we need to pass this other ID, >> + * so the guest can use it as the inode ID until the submount is >> + * auto-mounted. (At which point the guest will invoke getattr and >> + * find the inode ID on the submount.)) >> + * >> + * Return 0 on success, and -errno otherwise. *pino is set only in >> + * case of success. >> + */ >> +static int get_mp_ino_on_parent(const struct lo_inode *dir, const char *mp_name, >> + ino_t *pino) >> +{ >> + int dirfd = -1; >> + int ret; >> + DIR *dp = NULL; >> + >> + dirfd = openat(dir->fd, ".", O_RDONLY); >> + if (dirfd < 0) { >> + ret = -errno; >> + goto out; >> + } >> + >> + dp = fdopendir(dirfd); >> + if (!dp) { >> + ret = -errno; >> + goto out; >> + } >> + /* Owned by dp now */ >> + dirfd = -1; >> + >> + while (true) { >> + struct dirent *de; >> + >> + errno = 0; >> + de = readdir(dp); >> + if (!de) { >> + ret = errno ? -errno : -ENOENT; >> + goto out; >> + } >> + >> + if (!strcmp(de->d_name, mp_name)) { >> + *pino = de->d_ino; >> + ret = 0; >> + goto out; >> + } >> + } >> + >> +out: >> + if (dp) { >> + closedir(dp); >> + } >> + if (dirfd >= 0) { >> + close(dirfd); >> + } >> + return ret; >> +} >> + >> /* >> * Increments nlookup on the inode on success. unref_inode_lolocked() must be >> * called eventually to decrement nlookup again. If inodep is non-NULL, the >> * inode pointer is stored and the caller must call lo_inode_put(). >> + * >> + * If parent_fs_st_ino is true, the entry is a mount point, and submounts are >> + * announced to the guest, set e->attr.st_ino to the entry's inode ID on its >> + * parent filesystem instead of its inode ID on the filesystem mounted on it. >> + * (For mount points, the entry encompasses two inodes: One on the parent FS, >> + * and one on the mounted FS (where it is the root node), so it has two inode >> + * IDs. When looking up entries, we should show the guest the parent FS's inode >> + * ID, because as long as the guest has not auto-mounted the submount, it should >> + * see that original ID. Once it does perform the auto-mount, it will invoke >> + * getattr and see the root node's inode ID.) >> */ >> static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, >> struct fuse_entry_param *e, >> - struct lo_inode **inodep) >> + struct lo_inode **inodep, >> + bool parent_fs_st_ino) >> { >> int newfd; >> int res; >> @@ -984,6 +1057,7 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, >> struct lo_data *lo = lo_data(req); >> struct lo_inode *inode = NULL; >> struct lo_inode *dir = lo_inode(req, parent); >> + ino_t ino_id_for_guest; >> >> if (inodep) { >> *inodep = NULL; /* in case there is an error */ >> @@ -1018,9 +1092,22 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, >> goto out_err; >> } >> >> + ino_id_for_guest = e->attr.st_ino; >> + >> if (S_ISDIR(e->attr.st_mode) && lo->announce_submounts && >> (e->attr.st_dev != dir->key.dev || mnt_id != dir->key.mnt_id)) { >> e->attr_flags |= FUSE_ATTR_SUBMOUNT; >> + >> + if (parent_fs_st_ino) { >> + /* >> + * Best effort, so ignore errors. >> + * Also note that using readdir() means there may be races: >> + * The directory entry we find (if any) may be different >> + * from newfd. Again, this is a best effort. Reporting >> + * the wrong inode ID to the guest is not catastrophic. >> + */ >> + get_mp_ino_on_parent(dir, name, &ino_id_for_guest); > > Hi Max, > > [CC virtio-fs list ] > > In general patch looks good to me. A minor nit. get_mp_ino_on_parent() > is retruning error. It might be better to capture error and print a > message and continue. Sure, why not. > I have couple of general questions about submounts. > > - What happens in case of single file mounted on top of another file. > > mount --bind foo.txt bar.txt > > Do submounts work when mount point is not a directory. No, as you can see in the condition quoted above, we only set the FUSE_ATTR_SUBMOUNT flag for directories. That seemed the most common case for me, and I didn’t want to have to worry about weirdness that might ensue for file mounts. > - Say a directory is not a mount point yet and lookup instantiates an > inode. Later user mounts something on that directory. When does > client/server notice this change. I am assuming this is probably > part of revalidation path. I guess at least before this patch this is no different from any other filesystem change. Because st_dev+st_ino changed, it should basically look like the old directory was removed and a different one was put in its place. Now, with this patch, we will return the old st_ino to the guest, but internally virtiofsd will still use the submount’s st_dev/st_ino, so a new lo_inode should be created, and so fuse_dentry_revalidate()’s lookup should return a different node ID, resulting it to consider the entry expired. Besides, fuse_dentry_revalidate() has a condition on IS_AUTOMOUNT(inode) != flags & FUSE_ATTR_SUBMOUNT. Considering the previous paragraph, this doesn’t seem necessary to me, but it can’t hurt. Max