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[2003:cf:d711:bb59:b996:2e0b:622e:25cc]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a9ee0e2f0b5sm70060566b.192.2024.11.07.02.00.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 Nov 2024 02:00:40 -0800 (PST) Message-ID: Date: Thu, 7 Nov 2024 11:00:39 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] virtio-fs: Query rootmode during mount To: Miklos Szeredi Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, virtualization@lists.linux.dev, Miklos Szeredi , German Maglione , Stefan Hajnoczi , =?UTF-8?Q?Eugenio_P=C3=A9rez?= , Vivek Goyal References: <20241024164726.77485-1-hreitz@redhat.com> Content-Language: en-US From: Hanna Czenczek In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 07.11.24 09:58, Miklos Szeredi wrote: > On Thu, 24 Oct 2024 at 18:47, Hanna Czenczek wrote: > >> To be able to issue INIT (and GETATTR), we need to at least partially >> initialize the super_block structure, which is currently done via >> fuse_fill_super_common(). > What exactly is needed to be initialized? It isn’t much, but I believe it’s most of fuse_fill_super_common() (without restructuring the code so flags returned by INIT are put into a separate structure and then re-joined into sb and fc later). fuse_send_init() reads sb->s_bdi->ra_pages, process_init_reply() writes it and sb->s_time_gran, ->s_flags, ->s_stack_depth, ->s_export_op, and ->s_iflags.  In addition, process_init_reply() depends on several flags and objects in fc being set up (among those are fc->dax and fc->default_permissions), which is done by fuse_fill_super_common(). So I think what we need from fuse_fill_super_common() is: - fuse_sb_defaults() (so these values can then be overwritten by process_init_reply()), - fuse_dax_conn_alloc(), - fuse_bdi_init(), - fc->default_permissions at least, but I’d just take the fc->[flag] setting block as a whole then. I assume we’ll also want the SB_MANDLOCK check then, and rcu_assign_pointer().  Then we might as well also set the block sizes and the subtype. The problem is that I don’t know the order things in fuse_fill_super_common() need to be in, and fuse_dev_alloc_install() is called before fuse_bdi_init(), so I didn’t want to move that. So what I understand is that calling fuse_dev_alloc_install() there isn’t necessary?  I’m happy to move that to part 2, as you suggest, but I’m not sure we can really omit much from part 1 without changing how process_init_reply() operates.  We could in theory delay process_init_reply() until after GETATTR (and thus after setting s_root), but that seems kind of wrong, and would still require setting up BDI and DAX for fuse_send_init(). >> @@ -1762,18 +1801,12 @@ int fuse_fill_super_common(struct super_block *sb, struct fuse_fs_context *ctx) >> sb->s_d_op = &fuse_dentry_operations; >> >> mutex_lock(&fuse_mutex); >> - err = -EINVAL; >> - if (ctx->fudptr && *ctx->fudptr) >> - goto err_unlock; >> - >> err = fuse_ctl_add_conn(fc); >> if (err) >> goto err_unlock; >> >> list_add_tail(&fc->entry, &fuse_conn_list); >> sb->s_root = root_dentry; >> - if (ctx->fudptr) >> - *ctx->fudptr = fud; > This is wrong, because we need the fuse_mutex protection for checking > and setting the private_data on the fuse device file. > > If this split is needed (which I'm not sure) then fud allocation > should probably be moved to part2 instead of moving the *ctx->fudptr > setup to part1. > > >> @@ -1635,8 +1657,16 @@ static void virtio_kill_sb(struct super_block *sb) >> struct fuse_mount *fm = get_fuse_mount_super(sb); >> bool last; >> >> - /* If mount failed, we can still be called without any fc */ >> - if (sb->s_root) { >> + /* >> + * Only destroy the connection after full initialization, i.e. >> + * once s_root is set (see commit d534d31d6a45d). >> + * One exception: For virtio-fs, we call INIT before s_root is >> + * set so we can determine the root node's mode. We must call >> + * DESTROY after INIT. So if an error occurs during that time >> + * window (specifically in fuse_make_root_inode()), we still >> + * need to call virtio_fs_conn_destroy() here. >> + */ >> + if (sb->s_root || (fm->fc && fm->fc->initialized && !fm->submount)) { > How could fm->submount be set if sb->s_root isn't? fuse_get_tree_submount(), specifically fuse_fill_super_submount() whose error path leads to deactivate_locked_super(), can fail before sb->s_root is set. Still, the idea was rather to make it clear that this condition (INIT sent but s_root not set) is unique to non-submounts, so as not to mess with the submount code unintentionally. > Or sb->s_root set > and fc->initialized isn't? That would be the non-virtio-fs non-submount case (fuse_fill_super()), where s_root is set first and INIT sent after. Hanna > Seems it would be sufficient to check fm->fc->initialized, no? > > Thanks, > Miklos >