Re: [Qemu-devel] [PATCH v9 16/17] block/io_uring: adds fd registration

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Aug 02, 2019 at 05:10:30AM +0530, Aarushi Mehta wrote:

The fd lifecycle/leak issue remains.  After a drive is removed the
kernel still has a reference to the file.  If this repeats many times
our process will run out of open files.

A callback is required to unregister the file descriptor in
block/file-posix.c:

  static void raw_aio_detach_aio_context(BlockDriverState *bs)
  {
  #ifdef CONFIG_LINUX_IO_URING
      BDRVRawState *s = bs->opaque;
      LuringState *luring;

      luring = aio_get_linux_io_uring(bdrv_get_aio_context(bs));

      if (luring && s->fd >= 0) {
          luring_fd_unregister(luring, s->fd);
      }
  #endif
  }

I think this should eliminate fd leaks, but please test it.  You can use
drive_add/drive_del and device_add/device_del to hotplug and unplug
-drive and -device objects on the HMP monitor.  Use "ls -l /proc/PID/fd"
to see the list of currently open files.

> +    g_hash_table_insert(lookup, GINT_TO_POINTER(fd), GINT_TO_POINTER(nr));
> +    trace_luring_fd_register(fd, nr);

This trace event can be made even more useful by including
io_uring_register_files()'s return value so we know whether the kernel
accepted fd_array[] or not.

> +    return io_uring_register_files(ring, fd_reg->fd_array, nr + 1);
> +}
> +/**
> + * luring_fd_unregister:
> + *
> + * Unregisters file descriptors, TODO: error handling
> + */
> +static void luring_fd_unregister(LuringState *s)
> +{
> +        io_uring_unregister_files(&s->ring);
> +        g_hash_table_unref(s->fd_reg.fd_lookup);
> +        g_free(s->fd_reg.fd_array);

Please use 4-space indentation.

Missing s->fd_reg.fd_array = NULL so that the next g_realloc_n()
allocates a fresh array instead of trying to reallocate a freed pointer.

> +}
> +
> +/**
> + * luring_fd_lookup:
> + *
> + * Used to lookup fd index in registered array at submission time
> + * If the lookup table has not been created or the fd is not in the table,
> + * the fd is registered.
> + *
> + * If registration errors, the hash is cleared and the fd used directly
> + *
> + * Unregistering is done at luring_detach_aio_context
> + */
> +static int luring_fd_lookup(LuringState *s, int fd)
> +{
> +    int ret;
> +    void *index;
> +    GHashTable *lookup;
> +
> +    if (!s->fd_reg.fd_lookup) {
> +        s->fd_reg.fd_lookup = g_hash_table_new_full(g_direct_hash,
> +                                                    g_direct_equal,
> +                                                    g_free, g_free);
> +        luring_fd_register(&s->ring, &s->fd_reg, fd);
> +    }

This if statement can be eliminated:

  static void luring_fd_init(LuringState *s)
  {
      s->fd_reg.fd_lookup = g_hash_table_new_full(g_direct_hash,
                                                  g_direct_equal,
						  g_free, g_free);
  }

  static void luring_fd_cleanup(LuringState *s)
  {
      io_uring_unregister_files(&s->ring);
      g_hash_table_unref(s->fd_reg.fd_lookup);
      g_free(s->fd_reg.fd_array);
      s->fd_reg.fd_array = NULL;
  }

Call luring_fd_init() from luring_attach_aio_context() and call
luring_fd_cleanup() from luring_detach_aio_context().  This makes
luring_fd_lookup() simpler and gives a nice symmetry to attach/detach.

luring_fd_cleanup() is just luring_fd_unregister() renamed.

> +    lookup = s->fd_reg.fd_lookup;
> +    index = g_hash_table_lookup(lookup, GINT_TO_POINTER(fd));
> +
> +    if (index < 0) {
> +        ret = luring_fd_register(&s->ring, &s->fd_reg, fd);
> +
> +        if (ret < 0) {
> +            if (ret == -ENOMEM || ret == -EMFILE ||
> +                ret == -ENXIO) {
> +                return ret;
> +            } else {
> +                /* Should not reach here */
> +                g_hash_table_remove_all(lookup);
> +                g_free(s->fd_reg.fd_array);
> +                return ret;

I suggest making luring_fd_register() clean up after itself when an
error occurs.  Then you can change this code to:

  if (ret < 0) {
      return ret;
  }

It's usually convenient for a function to clean up after itself instead
of relying on the caller to do it since only the function knows exactly
what state has been modified so far.

The luring_fd_register() code becomes:

  ret = io_uring_register_files(ring, fd_reg->fd_array, nr + 1);
  if (ret == -ENOMEM || ret == -EMFILE || ret == -ENXIO) {
      /* Leave fd_array[] alone, fd will be overwritten next time anyway */
      g_hash_table_remove(lookup, GINT_TO_POINTER(fd));
  } else if (ret < 0) {
      /* A more severe error, clear out all registered fds */
      g_hash_table_remove_all(lookup);
      g_free(s->fd_reg.fd_array);
      s->fd_reg.fd_array = NULL;
  }
  return ret;

> +            }
> +        }
> +        index = g_hash_table_lookup(lookup, GINT_TO_POINTER(fd));

One final idea: make luring_fd_register() return the index on success so
callers don't need to look up the key again.  In luring_fd_register():

  if (ret < 0) {
      return ret;
  } else {
      return nr;
  }

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