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

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> 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.
>

That's QIOTask weirdness isn't it? It will run the worker in the thread,
but it also schedules the completion function as a glib event. So that's
when multifd_new_send_channel_async() will run. The crucial aspect here
is that it gets dispatched by glib on the main loop. I'm just keeping
the model, except that I don't have work to do during the "connection"
phase.

> 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.

It's async regarding multifd_channel_connect(). The connections will be
happening while multifd_save_setup() continues execution, IIUC.

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

I don't see the point in moving any of that logic into the URI
implementation. We already have the TLS handshake code which can also
call multifd_channel_connect() and that is a mess. IMO we should be
keeping the interface between multifd and the frontends as boilerplate
as possible.

>> +}
>> +
>>  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?
>

Possibly the other way around as you mention. I'll take a look.

>> +
>>  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
>>