On Tue, Feb 15, 2022 at 10:18:03AM +0100, Greg Kurz wrote: > On Mon, 14 Feb 2022 14:09:47 -0500 > Vivek Goyal <vgo...@redhat.com> wrote: > > > On Mon, Feb 14, 2022 at 01:56:08PM -0500, Vivek Goyal wrote: > > > On Mon, Feb 14, 2022 at 01:27:22PM -0500, Vivek Goyal wrote: > > > > On Mon, Feb 14, 2022 at 02:58:20PM +0100, Greg Kurz wrote: > > > > > This adds the missing bits to support FUSE_SYNCFS in the case > > > > > submounts > > > > > aren't announced to the client. > > > > > > > > > > Iterate over all inodes and call syncfs() on the ones marked as > > > > > submounts. > > > > > Since syncfs() can block for an indefinite time, we cannot call it > > > > > with > > > > > lo->mutex held as it would prevent the server to process other > > > > > requests. > > > > > This is thus broken down in two steps. First build a list of submounts > > > > > with lo->mutex held, drop the mutex and finally process the list. A > > > > > reference is taken on the inodes to ensure they don't go away when > > > > > lo->mutex is dropped. > > > > > > > > > > Signed-off-by: Greg Kurz <gr...@kaod.org> > > > > > --- > > > > > tools/virtiofsd/passthrough_ll.c | 38 > > > > > ++++++++++++++++++++++++++++++-- > > > > > 1 file changed, 36 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/tools/virtiofsd/passthrough_ll.c > > > > > b/tools/virtiofsd/passthrough_ll.c > > > > > index e94c4e6f8635..7ce944bfe2a0 100644 > > > > > --- a/tools/virtiofsd/passthrough_ll.c > > > > > +++ b/tools/virtiofsd/passthrough_ll.c > > > > > @@ -3400,8 +3400,42 @@ static void lo_syncfs(fuse_req_t req, > > > > > fuse_ino_t ino) > > > > > err = lo_do_syncfs(lo, inode); > > > > > lo_inode_put(lo, &inode); > > > > > } else { > > > > > - /* Requires the sever to track submounts. Not implemented > > > > > yet */ > > > > > - err = ENOSYS; > > > > > + g_autoptr(GSList) submount_list = NULL; > > > > > + GSList *elem; > > > > > + GHashTableIter iter; > > > > > + gpointer key, value; > > > > > + > > > > > + pthread_mutex_lock(&lo->mutex); > > > > > + > > > > > + g_hash_table_iter_init(&iter, lo->inodes); > > > > > + while (g_hash_table_iter_next(&iter, &key, &value)) { > > > > > > > > Going through all the inodes sounds very inefficient. If there are large > > > > number of inodes (say 1 million or more), and if frequent syncfs > > > > requests > > > > are coming this can consume lot of cpu cycles. > > > > > > > > Given C virtiofsd is slowly going away, so I don't want to be too > > > > particular about it. But, I would have thought to put submount > > > > inodes into another list or hash map (using mount id as key) and just > > > > traverse through that list instead. Given number of submounts should > > > > be small, it should be pretty quick to walk through that list. > > > > > > > > > + struct lo_inode *inode = value; > > > > > + > > > > > + if (inode->is_submount) { > > > > > + g_atomic_int_inc(&inode->refcount); > > > > > + submount_list = g_slist_prepend(submount_list, > > > > > inode); > > > > > + } > > > > > + } > > > > > + > > > > > + pthread_mutex_unlock(&lo->mutex); > > > > > + > > > > > + /* The root inode is always present and not tracked in the > > > > > hash table */ > > > > > + err = lo_do_syncfs(lo, &lo->root); > > > > > + > > > > > + for (elem = submount_list; elem; elem = g_slist_next(elem)) { > > > > > + struct lo_inode *inode = elem->data; > > > > > + int r; > > > > > + > > > > > + r = lo_do_syncfs(lo, inode); > > > > > + if (r) { > > > > > + /* > > > > > + * Try to sync as much as possible. Only one error > > > > > can be > > > > > + * reported to the client though, arbitrarily the > > > > > last one. > > > > > + */ > > > > > + err = r; > > > > > + } > > > > > + lo_inode_put(lo, &inode); > > > > > + } > > > > > > > > One more minor nit. What happens if virtiofsd is processing syncfs list > > > > and then somebody hard reboots qemu and mounts virtiofs again. That > > > > will trigger FUSE_INIT and will call lo_destroy() first. > > > > > > > > fuse_lowlevel.c > > > > > > > > fuse_session_process_buf_int() > > > > { > > > > fuse_log(FUSE_LOG_DEBUG, "%s: reinit\n", __func__); > > > > se->got_destroy = 1; > > > > se->got_init = 0; > > > > if (se->op.destroy) { > > > > se->op.destroy(se->userdata); > > > > } > > > > } > > > > > > > > IIUC, there is no synchronization with this path. If we are running with > > > > thread pool enabled, it could very well happen that one thread is still > > > > doing syncfs while other thread is executing do_init(). That sounds > > > > like little bit of a problem. It will be good if there is a way > > > > to either abort syncfs() or do_destroy() waits for all the previous > > > > syncfs() to finish. > > > > > > > > Greg, if you like, you could break down this work in two patch series. > > > > First patch series just issues syncfs() on inode id sent with > > > > FUSE_SYNCFS. > > > > That's easy fix and can get merged now. > > > > > > Actually I think even single "syncfs" will have synchronization issue > > > with do_init() upon hard reboot if we drop lo->mutex during syncfs(). > > > > Actually, we have similar issues with ->fsync(). We take lo->mutex, > > and then take a reference on inode. Call fsync() on this. Now it is > > possible that guest hard reboots, triggers, FUSE_INIT and lo_destroy() > > is called. It will take lo->mutex and drop its referene on inode. > > > > So it looks like in extreme case a new connection can start looking > > up inodes which we still have old inodes in hash table because > > some thread is blocked doing operation and has not dropped its > > reference. > > > > David, do I understand it right? > > > > We probably need to have a notion of keeping track of number of requests > > which are in progress. And lo_destroy() should wait till number of > > requests in progress come to zero. This will be equivalent of draining > > the queues operation in virtiofs kernel driver. > > > > Anyway, given we already have the issue w.r.t lo_destroy(), and C code > > is going away, I will be fine even if you don't fix races with FUSE_INIT. > > > > Vivek > > As you pointed out, this can affect other type of requests as well, so > this would probably deserve a more generic fix than just making it > work for syncfs(). This would most likely call for cycles that I don't > have. Thanks ! ;-) > > BTW, does the rust implementation have the same flaw ?
I don't think Rust implementation drops any locks at all while syncfs() is called. So next FUSE_INIT might just serialize completely and wait for syncfs() to finish first. But don't quote me on this because I don't understand rust virtiofsd locking well yet. It is more of a guess. Vivek > > > > > > > Vivek > > > > > > > > > > > And second patch series take care of above issues and will be little bit > > > > more work. > > > > > > > > Thanks > > > > Vivek > > > _______________________________________________ Virtio-fs mailing list Virtio-fs@redhat.com https://listman.redhat.com/mailman/listinfo/virtio-fs