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

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 29, 2024 at 02:34:13PM +0000, Daniel P. Berrangé wrote:
> 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;

I see.  Let's add a comment to explain why we do this explicit close(),
then?  It wasn't that clear, and we also don't do that on recv side.  It
seems also only useful for "file:", we can mention that in the comment if
so.

Thanks,

-- 
Peter Xu