Re: [PATCH v5 17/23] migration/multifd: Add outgoing QIOChannelFile support

[Daniel P . Berrangé]

On Thu, Feb 29, 2024 at 11:27:44AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Thu, Feb 29, 2024 at 10:44:21AM +0800, Peter Xu wrote:
> >> On Wed, Feb 28, 2024 at 12:21:21PM -0300, Fabiano Rosas wrote:
> >> > Allow multifd to open file-backed channels. This will be used when
> >> > enabling the mapped-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.
> >> > 
> >> > Contrary to the socket migration, the file migration doesn't need an
> >> > asynchronous channel creation process, so expose
> >> > multifd_channel_connect() and call it directly.
> >> > 
> >> > 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>
> >> > ---
> >> > - moved flags change to another patch
> >> > - removed channels_created assert
> >> > ---
> >> >  migration/file.c    | 41 +++++++++++++++++++++++++++++++++++++++--
> >> >  migration/file.h    |  4 ++++
> >> >  migration/multifd.c | 18 +++++++++++++++---
> >> >  migration/multifd.h |  1 +
> >> >  4 files changed, 59 insertions(+), 5 deletions(-)
> >> > 
> >> > diff --git a/migration/file.c b/migration/file.c
> >> > index 22d052a71f..83328a7a1b 100644
> >> > --- a/migration/file.c
> >> > +++ b/migration/file.c
> >> > @@ -12,12 +12,17 @@
> >> >  #include "channel.h"
> >> >  #include "file.h"
> >> >  #include "migration.h"
> >> > +#include "multifd.h"
> >> >  #include "io/channel-file.h"
> >> >  #include "io/channel-util.h"
> >> >  #include "trace.h"
> >> >  
> >> >  #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)
> >> > @@ -37,6 +42,36 @@ int file_parse_offset(char *filespec, uint64_t 
> >> > *offsetp, Error **errp)
> >> >      return 0;
> >> >  }
> >> >  
> >> > +void file_cleanup_outgoing_migration(void)
> >> > +{
> >> > +    g_free(outgoing_args.fname);
> >> > +    outgoing_args.fname = NULL;
> >> > +}
> >> > +
> >> > +bool file_send_channel_create(gpointer opaque, Error **errp)
> >> > +{
> >> > +    QIOChannelFile *ioc;
> >> > +    int flags = O_WRONLY;
> >> > +    bool ret = true;
> >> > +
> >> > +    ioc = qio_channel_file_new_path(outgoing_args.fname, flags, 0, 
> >> > errp);
> >> > +    if (!ioc) {
> >> > +        ret = false;
> >> > +        goto out;
> >> > +    }
> >> > +
> >> > +    multifd_channel_connect(opaque, QIO_CHANNEL(ioc));
> >> > +
> >> > +out:
> >> > +    /*
> >> > +     * File channel creation is synchronous. However posting this
> >> > +     * semaphore here is simpler than adding a special case.
> >> > +     */
> >> > +    multifd_send_channel_created();
> >> > +
> >> > +    return ret;
> >> > +}
> >> > +
> >> >  void file_start_outgoing_migration(MigrationState *s,
> >> >                                     FileMigrationArgs *file_args, Error 
> >> > **errp)
> >> >  {
> >> > @@ -47,12 +82,14 @@ void file_start_outgoing_migration(MigrationState *s,
> >> >  
> >> >      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, O_CREAT | O_TRUNC | 
> >> > O_WRONLY,
> >> > +                                     0660, errp);
> >> 
> >> It seems this is still leftover?
> >> 
> >> >      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..4577f9efdd 100644
> >> > --- a/migration/file.h
> >> > +++ b/migration/file.h
> >> > @@ -9,10 +9,14 @@
> >> >  #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_cleanup_outgoing_migration(void);
> >> > +bool file_send_channel_create(gpointer opaque, Error **errp);
> >> >  #endif
> >> > diff --git a/migration/multifd.c b/migration/multifd.c
> >> > index 3574fd3953..f155223303 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"
> >> >  
> >> > @@ -694,6 +696,7 @@ static bool 
> >> > multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
> >> >  {
> >> >      if (p->c) {
> >> >          migration_ioc_unregister_yank(p->c);
> >> > +        qio_channel_close(p->c, NULL);
> >> 
> >> s/NULL/&error_abort/?
> >
> > Or we can drop this line?  IIUC iochannel finalize() will always close it,
> > or it could be a separate bug.
> >
> 
> We need it so the fsync happens. The finalize() will be a noop because
> the qio_channel_file_close() will clear the fd. Not the cleanest, but it
> works.

It is always wise to explicitly call 'close'.

If something still has a GSource watch registered against
the QIOChannel, that GSource will be holding a reference
on the QIOChannel and will thus prevent finalize() ever
running.

By calling close() you guarantee the channel is closed,
even if you've mistakenly leaked a GSource somewhere.
Finalize still won't run in that case, but at least the
FD is gone, and the HUP might cause the GSource callback
to trigger correct cleanup;

With regards,
Daniel
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