Re: [PATCH v4 1/2] virtiofsd: Track mounts

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Jan 25, 2022 at 03:12:11PM +0100, Greg Kurz wrote:
> The upcoming implementation of ->sync_fs() needs to know about all
> submounts in order to call syncfs() on them when virtiofsd is started
> without '-o announce_submounts'.
> 
> Track every inode that comes up with a new mount id in a GHashTable.
> If the mount id isn't available, e.g. no statx() on the host, fallback
> on the device id for the key. This is done during lookup because we
> only care for the submounts that the client knows about. The inode
> is removed from the hash table when ultimately unreferenced. This
> can happen on a per-mount basis when the client posts a FUSE_FORGET
> request or for all submounts at once with FUSE_DESTROY.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>
> ---
>  tools/virtiofsd/passthrough_ll.c | 43 +++++++++++++++++++++++++++++---
>  1 file changed, 40 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> index 64b5b4fbb186..7bf31fc129c8 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -117,6 +117,7 @@ struct lo_inode {
>      GHashTable *posix_locks; /* protected by lo_inode->plock_mutex */
>  
>      mode_t filetype;
> +    bool is_mnt;
>  };
>  
>  struct lo_cred {
> @@ -164,6 +165,7 @@ struct lo_data {
>      bool use_statx;
>      struct lo_inode root;
>      GHashTable *inodes; /* protected by lo->mutex */
> +    GHashTable *mnt_inodes; /* protected by lo->mutex */
>      struct lo_map ino_map; /* protected by lo->mutex */
>      struct lo_map dirp_map; /* protected by lo->mutex */
>      struct lo_map fd_map; /* protected by lo->mutex */
> @@ -1000,6 +1002,31 @@ static int do_statx(struct lo_data *lo, int dirfd, const char *pathname,
>      return 0;
>  }
>  

Hi Greg,

Thanks for the patches. Had a quick look. Overall these patches look
pretty good to me. I will spend more time testing and having a 
closer look. Some quick thoughts below.

> +static uint64_t mnt_inode_key(struct lo_inode *inode)
> +{
> +    /* Prefer mnt_id, fallback on dev */
> +    return inode->key.mnt_id ? inode->key.mnt_id : inode->key.dev;
> +}

I am not sure if we should use inode->key.dev. This might create problem
if same file system is bind mounted at two paths in shared dir. So
say /dev/sdb is mounted at foo1/ and then bind mounted at foo2/ in
shared dir. A user looks up foo1/ and does some writes. Then we
lookup foo2/ and release that inode. Release of foo2 will let go
inode from the hash. And that means if later another write happens
in foo1/ followed by syncfs(), we will not issue syncfs() on filesystem
backed by /dev/sdb.

So what are the options.

A. Make mnt_id mandatory and do not implement it if mnt_id is not
   available.

B. Don't do anything and live with this. It is a corner case and
   still better than not implement submount syncfs at all.

C. Instead of adding lo_inode to hash, create another kind of object
   and reference count that. It could be a mount fd which we open
   when we add object for the first time. So when foo1/ inode is
   instantiated, create mountfd object, add it to hash table using
   device id as the key. When foo2 comes along, we find the object
   in the hash and just bump up the ref. Now this mountfd object
   will go away when both foo1 and foo2 inodes have been evicted
   and will take care of the issue I am referring to.

I guess B is little extra complexity but probably not too bad.
WDYT. It sounds litter better than option A and B.


> +
> +static void add_mnt_inode(struct lo_data *lo, struct lo_inode *inode)
> +{
> +    uint64_t mnt_key = mnt_inode_key(inode);
> +
> +    if (!g_hash_table_contains(lo->mnt_inodes, &mnt_key)) {
> +        inode->is_mnt = true;
> +        g_hash_table_insert(lo->mnt_inodes, &mnt_key, inode);
> +    }
> +}
> +
> +static void remove_mnt_inode(struct lo_data *lo, struct lo_inode *inode)
> +{
> +    uint64_t mnt_key = mnt_inode_key(inode);
> +
> +    if (inode->is_mnt) {
> +        g_hash_table_remove(lo->mnt_inodes, &mnt_key);
> +    }
> +}

Should we issue syncfs() on this inode when we are removing it? It
is possible guest did some writes, let go inode and later issued
a syncfs(). By that time inode is gone and we will not issue any
syncfs() on this filesystem. Hence leaving data in host page cache.

Thanks
Vivek

> +
>  /*
>   * Increments nlookup on the inode on success. unref_inode_lolocked() must be
>   * called eventually to decrement nlookup again. If inodep is non-NULL, the
> @@ -1086,10 +1113,15 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
>          pthread_mutex_lock(&lo->mutex);
>          inode->fuse_ino = lo_add_inode_mapping(req, inode);
>          g_hash_table_insert(lo->inodes, &inode->key, inode);
> +        add_mnt_inode(lo, inode);
>          pthread_mutex_unlock(&lo->mutex);
>      }
>      e->ino = inode->fuse_ino;
>  
> +    fuse_log(FUSE_LOG_DEBUG, "  %lli/%s -> %lli%s\n",
> +             (unsigned long long) parent, name, (unsigned long long) e->ino,
> +             inode->is_mnt ? " (submount)" : "");
> +
>      /* Transfer ownership of inode pointer to caller or drop it */
>      if (inodep) {
>          *inodep = inode;
> @@ -1099,9 +1131,6 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name,
>  
>      lo_inode_put(lo, &dir);
>  
> -    fuse_log(FUSE_LOG_DEBUG, "  %lli/%s -> %lli\n", (unsigned long long)parent,
> -             name, (unsigned long long)e->ino);
> -
>      return 0;
>  
>  out_err:
> @@ -1563,6 +1592,7 @@ static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n)
>              g_hash_table_destroy(inode->posix_locks);
>              pthread_mutex_destroy(&inode->plock_mutex);
>          }
> +        remove_mnt_inode(lo, inode);
>          /* Drop our refcount from lo_do_lookup() */
>          lo_inode_put(lo, &inode);
>      }
> @@ -3337,6 +3367,7 @@ static void lo_destroy(void *userdata)
>      struct lo_data *lo = (struct lo_data *)userdata;
>  
>      pthread_mutex_lock(&lo->mutex);
> +    g_hash_table_remove_all(lo->mnt_inodes);
>      while (true) {
>          GHashTableIter iter;
>          gpointer key, value;
> @@ -3850,6 +3881,7 @@ static void setup_root(struct lo_data *lo, struct lo_inode *root)
>          root->posix_locks = g_hash_table_new_full(
>              g_direct_hash, g_direct_equal, NULL, posix_locks_value_destroy);
>      }
> +    add_mnt_inode(lo, root);
>  }
>  
>  static guint lo_key_hash(gconstpointer key)
> @@ -3869,6 +3901,10 @@ static gboolean lo_key_equal(gconstpointer a, gconstpointer b)
>  
>  static void fuse_lo_data_cleanup(struct lo_data *lo)
>  {
> +    if (lo->mnt_inodes) {
> +        g_hash_table_destroy(lo->mnt_inodes);
> +    }
> +
>      if (lo->inodes) {
>          g_hash_table_destroy(lo->inodes);
>      }
> @@ -3931,6 +3967,7 @@ int main(int argc, char *argv[])
>      lo.root.fd = -1;
>      lo.root.fuse_ino = FUSE_ROOT_ID;
>      lo.cache = CACHE_AUTO;
> +    lo.mnt_inodes = g_hash_table_new(g_int64_hash, g_int64_equal);
>  
>      /*
>       * Set up the ino map like this:
> -- 
> 2.34.1
>