Re: [PATCH v6 1/1] virtiofsd: Add basic support for FUSE_SYNCFS request

[Vivek Goyal]

On Tue, Feb 15, 2022 at 07:15:29PM +0100, Greg Kurz wrote:
> Honor the expected behavior of syncfs() to synchronously flush all data
> and metadata to disk on linux systems.
> 
> If virtiofsd is started with '-o announce_submounts', the client is
> expected to send a FUSE_SYNCFS request for each individual submount.
> In this case, we just create a new file descriptor on the submount
> inode with lo_inode_open(), call syncfs() on it and close it. The
> intermediary file is needed because O_PATH descriptors aren't
> backed by an actual file and syncfs() would fail with EBADF.
> 
> If virtiofsd is started without '-o announce_submounts' or if the
> client doesn't have the FUSE_CAP_SUBMOUNTS capability, the client
> only sends a single FUSE_SYNCFS request for the root inode. The
> server would thus need to track submounts internally and call
> syncfs() on each of them. This will be implemented later.
> 
> Note that syncfs() might suffer from a time penalty if the submounts
> are being hammered by some unrelated workload on the host. The only
> solution to prevent that is to avoid shared mounts.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>

Looks good to me. Thanks Greg.

Reviewed-by: Vivek Goyal <vgoyal@redhat.com>

Vivek

> ---
>  tools/virtiofsd/fuse_lowlevel.c       | 11 +++++++
>  tools/virtiofsd/fuse_lowlevel.h       | 13 ++++++++
>  tools/virtiofsd/passthrough_ll.c      | 44 +++++++++++++++++++++++++++
>  tools/virtiofsd/passthrough_seccomp.c |  1 +
>  4 files changed, 69 insertions(+)
> 
> diff --git a/tools/virtiofsd/fuse_lowlevel.c b/tools/virtiofsd/fuse_lowlevel.c
> index e4679c73abc2..e02d8b25a5f6 100644
> --- a/tools/virtiofsd/fuse_lowlevel.c
> +++ b/tools/virtiofsd/fuse_lowlevel.c
> @@ -1876,6 +1876,16 @@ static void do_lseek(fuse_req_t req, fuse_ino_t nodeid,
>      }
>  }
>  
> +static void do_syncfs(fuse_req_t req, fuse_ino_t nodeid,
> +                      struct fuse_mbuf_iter *iter)
> +{
> +    if (req->se->op.syncfs) {
> +        req->se->op.syncfs(req, nodeid);
> +    } else {
> +        fuse_reply_err(req, ENOSYS);
> +    }
> +}
> +
>  static void do_init(fuse_req_t req, fuse_ino_t nodeid,
>                      struct fuse_mbuf_iter *iter)
>  {
> @@ -2280,6 +2290,7 @@ static struct {
>      [FUSE_RENAME2] = { do_rename2, "RENAME2" },
>      [FUSE_COPY_FILE_RANGE] = { do_copy_file_range, "COPY_FILE_RANGE" },
>      [FUSE_LSEEK] = { do_lseek, "LSEEK" },
> +    [FUSE_SYNCFS] = { do_syncfs, "SYNCFS" },
>  };
>  
>  #define FUSE_MAXOP (sizeof(fuse_ll_ops) / sizeof(fuse_ll_ops[0]))
> diff --git a/tools/virtiofsd/fuse_lowlevel.h b/tools/virtiofsd/fuse_lowlevel.h
> index c55c0ca2fc1c..b889dae4de0e 100644
> --- a/tools/virtiofsd/fuse_lowlevel.h
> +++ b/tools/virtiofsd/fuse_lowlevel.h
> @@ -1226,6 +1226,19 @@ struct fuse_lowlevel_ops {
>       */
>      void (*lseek)(fuse_req_t req, fuse_ino_t ino, off_t off, int whence,
>                    struct fuse_file_info *fi);
> +
> +    /**
> +     * Synchronize file system content
> +     *
> +     * If this request is answered with an error code of ENOSYS,
> +     * this is treated as success and future calls to syncfs() will
> +     * succeed automatically without being sent to the filesystem
> +     * process.
> +     *
> +     * @param req request handle
> +     * @param ino the inode number
> +     */
> +    void (*syncfs)(fuse_req_t req, fuse_ino_t ino);
>  };
>  
>  /**
> diff --git a/tools/virtiofsd/passthrough_ll.c 
> b/tools/virtiofsd/passthrough_ll.c
> index b3d0674f6d2f..0f65e6423cf5 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -3357,6 +3357,49 @@ static void lo_lseek(fuse_req_t req, fuse_ino_t ino, 
> off_t off, int whence,
>      }
>  }
>  
> +static int lo_do_syncfs(struct lo_data *lo, struct lo_inode *inode)
> +{
> +    int fd, ret = 0;
> +
> +    fuse_log(FUSE_LOG_DEBUG, "lo_do_syncfs(ino=%" PRIu64 ")\n",
> +             inode->fuse_ino);
> +
> +    fd = lo_inode_open(lo, inode, O_RDONLY);
> +    if (fd < 0) {
> +        return -fd;
> +    }
> +
> +    if (syncfs(fd) < 0) {
> +        ret = errno;
> +    }
> +
> +    close(fd);
> +    return ret;
> +}
> +
> +static void lo_syncfs(fuse_req_t req, fuse_ino_t ino)
> +{
> +    struct lo_data *lo = lo_data(req);
> +    struct lo_inode *inode = lo_inode(req, ino);
> +    int err;
> +
> +    if (!inode) {
> +        fuse_reply_err(req, EBADF);
> +        return;
> +    }
> +
> +    err = lo_do_syncfs(lo, inode);
> +    lo_inode_put(lo, &inode);
> +
> +    /*
> +     * If submounts aren't announced, the client only sends a request to
> +     * sync the root inode. TODO: Track submounts internally and iterate
> +     * over them as well.
> +     */
> +
> +    fuse_reply_err(req, err);
> +}
> +
>  static void lo_destroy(void *userdata)
>  {
>      struct lo_data *lo = (struct lo_data *)userdata;
> @@ -3417,6 +3460,7 @@ static struct fuse_lowlevel_ops lo_oper = {
>      .copy_file_range = lo_copy_file_range,
>  #endif
>      .lseek = lo_lseek,
> +    .syncfs = lo_syncfs,
>      .destroy = lo_destroy,
>  };
>  
> diff --git a/tools/virtiofsd/passthrough_seccomp.c 
> b/tools/virtiofsd/passthrough_seccomp.c
> index a3ce9f898d2d..3e9d6181dc69 100644
> --- a/tools/virtiofsd/passthrough_seccomp.c
> +++ b/tools/virtiofsd/passthrough_seccomp.c
> @@ -108,6 +108,7 @@ static const int syscall_allowlist[] = {
>      SCMP_SYS(set_robust_list),
>      SCMP_SYS(setxattr),
>      SCMP_SYS(symlinkat),
> +    SCMP_SYS(syncfs),
>      SCMP_SYS(time), /* Rarely needed, except on static builds */
>      SCMP_SYS(tgkill),
>      SCMP_SYS(unlinkat),
> -- 
> 2.34.1
>