Re: [Virtio-fs] [PATCH v5 1/3] virtiofsd: Add support for FUSE_SYNCFS request with announce_submounts

[Greg Kurz]

On Mon, 14 Feb 2022 16:43:15 +0100
German Maglione <gmaglione@redhat.com> wrote:

> On Mon, Feb 14, 2022 at 3:00 PM Greg Kurz <groug@kaod.org> 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>
> > ---
> >  tools/virtiofsd/fuse_lowlevel.c       | 11 +++++++
> >  tools/virtiofsd/fuse_lowlevel.h       | 13 ++++++++
> >  tools/virtiofsd/passthrough_ll.c      | 45 +++++++++++++++++++++++++++
> >  tools/virtiofsd/passthrough_seccomp.c |  1 +
> >  4 files changed, 70 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..2bf5d40df531 100644
> > --- a/tools/virtiofsd/passthrough_ll.c
> > +++ b/tools/virtiofsd/passthrough_ll.c
> > @@ -3357,6 +3357,50 @@ 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);
> > +    int err;
> > +
> > +    if (lo->announce_submounts) {
> > +        struct lo_inode *inode;
> > +
> > +        inode = lo_inode(req, ino);
> > +        if (!inode) {
> > +            fuse_reply_err(req, EBADF);
> > +            return;
> > +        }
> > +
> > +        err = lo_do_syncfs(lo, inode);
> > +        lo_inode_put(lo, &inode);
> > +    } else {
> > +        /* Requires the sever to track submounts. Not implemented yet */
> > +        err = ENOSYS;
> > +    }
> >
> 
> In the current rust version we don't check if announce_submount is enabled,
> with and without it lo_syncfs do the same thing. So, if announce_submount
> is not enabled, at least the root shared dir get synced.
> 

I hesitated to do that but since the announce_submount case is
handled in patch 3, I kept it simple. Anyway, I'm fine to post
a new version that matches what the rust implementation does.

> 
> 
> > +
> > +    fuse_reply_err(req, err);
> > +}
> > +
> >  static void lo_destroy(void *userdata)
> >  {
> >      struct lo_data *lo = (struct lo_data *)userdata;
> > @@ -3417,6 +3461,7 @@ 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
> >
> > _______________________________________________
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> >
> >
>