Re: [RFC PATCH v3 13/30] migration/multifd: Add outgoing QIOChannelFile support

[Peter Xu <peterx@redhat.com>]

On Mon, Nov 27, 2023 at 05:25:55PM -0300, Fabiano Rosas wrote:
> Allow multifd to open file-backed channels. This will be used when
> enabling the fixed-ram migration stream format which expects a
> seekable transport.
> 
> The QIOChannel read and write methods will use the preadv/pwritev
> versions which don't update the file offset at each call so we can
> reuse the fd without re-opening for every channel.
> 
> Note that this is just setup code and multifd cannot yet make use of
> the file channels.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
> - open multifd channels with O_WRONLY and no mode
> - stop cancelling migration and propagate error via qio_task
> ---
>  migration/file.c      | 47 +++++++++++++++++++++++++++++++++++++++++--
>  migration/file.h      |  5 +++++
>  migration/multifd.c   | 14 +++++++++++--
>  migration/options.c   |  7 +++++++
>  migration/options.h   |  1 +
>  migration/qemu-file.h |  1 -
>  6 files changed, 70 insertions(+), 5 deletions(-)
> 
> diff --git a/migration/file.c b/migration/file.c
> index 5d4975f43e..67d6f42da7 100644
> --- a/migration/file.c
> +++ b/migration/file.c
> @@ -17,6 +17,10 @@
>  
>  #define OFFSET_OPTION ",offset="
>  
> +static struct FileOutgoingArgs {
> +    char *fname;
> +} outgoing_args;
> +
>  /* Remove the offset option from @filespec and return it in @offsetp. */
>  
>  int file_parse_offset(char *filespec, uint64_t *offsetp, Error **errp)
> @@ -36,6 +40,42 @@ int file_parse_offset(char *filespec, uint64_t *offsetp, Error **errp)
>      return 0;
>  }
>  
> +static void qio_channel_file_connect_worker(QIOTask *task, gpointer opaque)
> +{
> +    /* noop */
> +}
> +
> +int file_send_channel_destroy(QIOChannel *ioc)
> +{
> +    if (ioc) {
> +        qio_channel_close(ioc, NULL);
> +        object_unref(OBJECT(ioc));
> +    }
> +    g_free(outgoing_args.fname);
> +    outgoing_args.fname = NULL;
> +
> +    return 0;
> +}
> +
> +void file_send_channel_create(QIOTaskFunc f, void *data)
> +{
> +    QIOChannelFile *ioc;
> +    QIOTask *task;
> +    Error *err = NULL;
> +    int flags = O_WRONLY;
> +
> +    ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, &err);
> +
> +    task = qio_task_new(OBJECT(ioc), f, (gpointer)data, NULL);
> +    if (!ioc) {
> +        qio_task_set_error(task, err);
> +        return;
> +    }
> +
> +    qio_task_run_in_thread(task, qio_channel_file_connect_worker,
> +                           (gpointer)data, NULL, NULL);

This is pretty weird.  This invokes a thread, but it'll run a noop.  It
seems meaningless to me.

I assume you wanted to keep using the same async model as the socket typed
multifd, but I don't think that works anyway, because file open blocks at
qio_channel_file_new_path() so it's sync anyway.

AFAICT we still share the code, as long as the file path properly invokes
multifd_channel_connect() after the iochannel is setup.

> +}
> +
>  void file_start_outgoing_migration(MigrationState *s,
>                                     FileMigrationArgs *file_args, Error **errp)
>  {
> @@ -43,15 +83,18 @@ void file_start_outgoing_migration(MigrationState *s,
>      g_autofree char *filename = g_strdup(file_args->filename);
>      uint64_t offset = file_args->offset;
>      QIOChannel *ioc;
> +    int flags = O_CREAT | O_TRUNC | O_WRONLY;
> +    mode_t mode = 0660;
>  
>      trace_migration_file_outgoing(filename);
>  
> -    fioc = qio_channel_file_new_path(filename, O_CREAT | O_WRONLY | O_TRUNC,
> -                                     0600, errp);
> +    fioc = qio_channel_file_new_path(filename, flags, mode, errp);
>      if (!fioc) {
>          return;
>      }
>  
> +    outgoing_args.fname = g_strdup(filename);
> +
>      ioc = QIO_CHANNEL(fioc);
>      if (offset && qio_channel_io_seek(ioc, offset, SEEK_SET, errp) < 0) {
>          return;
> diff --git a/migration/file.h b/migration/file.h
> index 37d6a08bfc..511019b319 100644
> --- a/migration/file.h
> +++ b/migration/file.h
> @@ -9,10 +9,15 @@
>  #define QEMU_MIGRATION_FILE_H
>  
>  #include "qapi/qapi-types-migration.h"
> +#include "io/task.h"
> +#include "channel.h"
>  
>  void file_start_incoming_migration(FileMigrationArgs *file_args, Error **errp);
>  
>  void file_start_outgoing_migration(MigrationState *s,
>                                     FileMigrationArgs *file_args, Error **errp);
>  int file_parse_offset(char *filespec, uint64_t *offsetp, Error **errp);
> +
> +void file_send_channel_create(QIOTaskFunc f, void *data);
> +int file_send_channel_destroy(QIOChannel *ioc);
>  #endif
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 123ff0dec0..427740aab6 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -17,6 +17,7 @@
>  #include "exec/ramblock.h"
>  #include "qemu/error-report.h"
>  #include "qapi/error.h"
> +#include "file.h"
>  #include "ram.h"
>  #include "migration.h"
>  #include "migration-stats.h"
> @@ -28,6 +29,7 @@
>  #include "threadinfo.h"
>  #include "options.h"
>  #include "qemu/yank.h"
> +#include "io/channel-file.h"
>  #include "io/channel-socket.h"
>  #include "yank_functions.h"
>  
> @@ -511,7 +513,11 @@ static void multifd_send_terminate_threads(Error *err)
>  
>  static int multifd_send_channel_destroy(QIOChannel *send)
>  {
> -    return socket_send_channel_destroy(send);
> +    if (migrate_to_file()) {
> +        return file_send_channel_destroy(send);
> +    } else {
> +        return socket_send_channel_destroy(send);
> +    }
>  }
>  
>  void multifd_save_cleanup(void)
> @@ -904,7 +910,11 @@ static void multifd_new_send_channel_async(QIOTask *task, gpointer opaque)
>  
>  static void multifd_new_send_channel_create(gpointer opaque)
>  {
> -    socket_send_channel_create(multifd_new_send_channel_async, opaque);
> +    if (migrate_to_file()) {
> +        file_send_channel_create(multifd_new_send_channel_async, opaque);
> +    } else {
> +        socket_send_channel_create(multifd_new_send_channel_async, opaque);
> +    }
>  }
>  
>  int multifd_save_setup(Error **errp)
> diff --git a/migration/options.c b/migration/options.c
> index 10730b13ba..f671e24758 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -409,6 +409,13 @@ bool migrate_tls(void)
>      return s->parameters.tls_creds && *s->parameters.tls_creds;
>  }
>  
> +bool migrate_to_file(void)
> +{
> +    MigrationState *s = migrate_get_current();
> +
> +    return qemu_file_is_seekable(s->to_dst_file);
> +}

Would this migrate_to_file() == migrate_multifd_packets()?

Maybe we can keep using the other one and drop migrate_to_file?

> +
>  typedef enum WriteTrackingSupport {
>      WT_SUPPORT_UNKNOWN = 0,
>      WT_SUPPORT_ABSENT,
> diff --git a/migration/options.h b/migration/options.h
> index 8a19d6939c..84628a76e8 100644
> --- a/migration/options.h
> +++ b/migration/options.h
> @@ -60,6 +60,7 @@ bool migrate_multifd_packets(void);
>  bool migrate_postcopy(void);
>  bool migrate_rdma(void);
>  bool migrate_tls(void);
> +bool migrate_to_file(void);
>  
>  /* capabilities helpers */
>  
> diff --git a/migration/qemu-file.h b/migration/qemu-file.h
> index 32fd4a34fd..78ea21ab98 100644
> --- a/migration/qemu-file.h
> +++ b/migration/qemu-file.h
> @@ -83,5 +83,4 @@ size_t qemu_get_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen,
>                            off_t pos);
>  
>  QIOChannel *qemu_file_get_ioc(QEMUFile *file);
> -
>  #endif
> -- 
> 2.35.3
> 

-- 
Peter Xu