Re: [PATCH v5 3/3] virtiofsd: Add support for FUSE_SYNCFS request without announce_submounts

[Vivek Goyal]

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 <groug@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().

Vivek

> 
> And second patch series take care of above issues and will be little bit
> more work.
> 
> Thanks
> Vivek