[2/2] migration/qemu-file: fix potential buf waste for extra buf_index adjustment
diff mbox series

Message ID 20190731144225.3784-3-richardw.yang@linux.intel.com
State New
Headers show
Series
  • migration/qemu-file: cleanup and refine qemu-file
Related show

Commit Message

Wei Yang July 31, 2019, 2:42 p.m. UTC
In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
this happens, buf_index is reset. Currently, this is not checked and
buf_index would always been adjust with buf size.

This is not harmful, but will waste some space in file buffer.

This patch make add_to_iovec() return 1 when it has flushed the file.
Then the caller could check the return value to see whether it is
necessary to adjust the buf_index any more.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

Comments

Dr. David Alan Gilbert Aug. 23, 2019, 11:06 a.m. UTC | #1
(Copying Dan in)

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> this happens, buf_index is reset. Currently, this is not checked and
> buf_index would always been adjust with buf size.
> 
> This is not harmful, but will waste some space in file buffer.

That's a nice find.

> This patch make add_to_iovec() return 1 when it has flushed the file.
> Then the caller could check the return value to see whether it is
> necessary to adjust the buf_index any more.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

(I wonder if there's a way to wrap that little add_to_iovec, check, add
to index, flush in a little wrapper).

Dave

> ---
>  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
>  1 file changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 35c22605dd..05d9f42ddb 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
>      return ret;
>  }
>  
> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> -                         bool may_free)
> +/*
> + * Add buf to iovec. Do flush if iovec is full.
> + *
> + * Return values:
> + * 1 iovec is full and flushed
> + * 0 iovec is not flushed
> + *
> + */
> +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> +                        bool may_free)
>  {
>      /* check for adjacent buffer and coalesce them */
>      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>  
>      if (f->iovcnt >= MAX_IOV_SIZE) {
>          qemu_fflush(f);
> +        return 1;
>      }
> +
> +    return 0;
>  }
>  
>  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>          }
>          memcpy(f->buf + f->buf_index, buf, l);
>          f->bytes_xfer += l;
> -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> -        f->buf_index += l;
> -        if (f->buf_index == IO_BUF_SIZE) {
> -            qemu_fflush(f);
> +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
> +            f->buf_index += l;
> +            if (f->buf_index == IO_BUF_SIZE) {
> +                qemu_fflush(f);
> +            }
>          }
>          if (qemu_file_get_error(f)) {
>              break;
> @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
>  
>      f->buf[f->buf_index] = v;
>      f->bytes_xfer++;
> -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> -    f->buf_index++;
> -    if (f->buf_index == IO_BUF_SIZE) {
> -        qemu_fflush(f);
> +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> +        f->buf_index++;
> +        if (f->buf_index == IO_BUF_SIZE) {
> +            qemu_fflush(f);
> +        }
>      }
>  }
>  
> @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>      }
>  
>      qemu_put_be32(f, blen);
> -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> -    f->buf_index += blen;
> -    if (f->buf_index == IO_BUF_SIZE) {
> -        qemu_fflush(f);
> +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
> +        f->buf_index += blen;
> +        if (f->buf_index == IO_BUF_SIZE) {
> +            qemu_fflush(f);
> +        }
>      }
>      return blen + sizeof(int32_t);
>  }
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Daniel P. Berrangé Aug. 23, 2019, 11:38 a.m. UTC | #2
On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
> (Copying Dan in)
> 
> * Wei Yang (richardw.yang@linux.intel.com) wrote:
> > In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> > this happens, buf_index is reset. Currently, this is not checked and
> > buf_index would always been adjust with buf size.
> > 
> > This is not harmful, but will waste some space in file buffer.
> 
> That's a nice find.
> 
> > This patch make add_to_iovec() return 1 when it has flushed the file.
> > Then the caller could check the return value to see whether it is
> > necessary to adjust the buf_index any more.
> > 
> > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> (I wonder if there's a way to wrap that little add_to_iovec, check, add
> to index, flush in a little wrapper).

Given the name "add_to_iovec" I think it is pretty surprising
that it calls "qemu_flush" at all.

It is also pretty wierd that we're checking two different
conditions in two different places.

Right now the code is essentially doing this:

     if (f->iovcnt >= MAX_IOV_SIZE) {
        qemu_fflush(f);
     }
     if (f->buf_index == IO_BUF_SIZE) {
        qemu_fflush(f);
     }

Except that in the qemu_put_buffer_async() case, we're
only doing the first of these two checks. This feels
very odd indeed - I would have thought either it should
do both, or do neither.

Assuming doing both flushs is ok for qemu_put_buffer_async
then I'd suggest renaming 'add_to_iovec' to 'queue_buffer'
and have that method do both of these qemu_fflush() calls.

> > ---
> >  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
> >  1 file changed, 28 insertions(+), 14 deletions(-)
> > 
> > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > index 35c22605dd..05d9f42ddb 100644
> > --- a/migration/qemu-file.c
> > +++ b/migration/qemu-file.c
> > @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
> >      return ret;
> >  }
> >  
> > -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > -                         bool may_free)
> > +/*
> > + * Add buf to iovec. Do flush if iovec is full.
> > + *
> > + * Return values:
> > + * 1 iovec is full and flushed
> > + * 0 iovec is not flushed
> > + *
> > + */
> > +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > +                        bool may_free)
> >  {
> >      /* check for adjacent buffer and coalesce them */
> >      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> > @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >  
> >      if (f->iovcnt >= MAX_IOV_SIZE) {
> >          qemu_fflush(f);
> > +        return 1;
> >      }
> > +
> > +    return 0;
> >  }
> >  
> >  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> > @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> >          }
> >          memcpy(f->buf + f->buf_index, buf, l);
> >          f->bytes_xfer += l;
> > -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> > -        f->buf_index += l;
> > -        if (f->buf_index == IO_BUF_SIZE) {
> > -            qemu_fflush(f);
> > +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
> > +            f->buf_index += l;
> > +            if (f->buf_index == IO_BUF_SIZE) {
> > +                qemu_fflush(f);
> > +            }
> >          }
> >          if (qemu_file_get_error(f)) {
> >              break;
> > @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
> >  
> >      f->buf[f->buf_index] = v;
> >      f->bytes_xfer++;
> > -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> > -    f->buf_index++;
> > -    if (f->buf_index == IO_BUF_SIZE) {
> > -        qemu_fflush(f);
> > +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> > +        f->buf_index++;
> > +        if (f->buf_index == IO_BUF_SIZE) {
> > +            qemu_fflush(f);
> > +        }
> >      }
> >  }
> >  
> > @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
> >      }
> >  
> >      qemu_put_be32(f, blen);
> > -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> > -    f->buf_index += blen;
> > -    if (f->buf_index == IO_BUF_SIZE) {
> > -        qemu_fflush(f);
> > +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
> > +        f->buf_index += blen;
> > +        if (f->buf_index == IO_BUF_SIZE) {
> > +            qemu_fflush(f);
> > +        }
> >      }
> >      return blen + sizeof(int32_t);
> >  }
> > -- 
> > 2.17.1
> > 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

Regards,
Daniel
Dr. David Alan Gilbert Aug. 23, 2019, 1:05 p.m. UTC | #3
* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
> > (Copying Dan in)
> > 
> > * Wei Yang (richardw.yang@linux.intel.com) wrote:
> > > In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> > > this happens, buf_index is reset. Currently, this is not checked and
> > > buf_index would always been adjust with buf size.
> > > 
> > > This is not harmful, but will waste some space in file buffer.
> > 
> > That's a nice find.
> > 
> > > This patch make add_to_iovec() return 1 when it has flushed the file.
> > > Then the caller could check the return value to see whether it is
> > > necessary to adjust the buf_index any more.
> > > 
> > > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> > 
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > 
> > (I wonder if there's a way to wrap that little add_to_iovec, check, add
> > to index, flush in a little wrapper).
> 
> Given the name "add_to_iovec" I think it is pretty surprising
> that it calls "qemu_flush" at all.
> 
> It is also pretty wierd that we're checking two different
> conditions in two different places.
> 
> Right now the code is essentially doing this:
> 
>      if (f->iovcnt >= MAX_IOV_SIZE) {
>         qemu_fflush(f);
>      }
>      if (f->buf_index == IO_BUF_SIZE) {
>         qemu_fflush(f);
>      }
> 
> Except that in the qemu_put_buffer_async() case, we're
> only doing the first of these two checks. This feels
> very odd indeed - I would have thought either it should
> do both, or do neither.

No, there's two separate types of buffers.

There's f->buf which is a single allocated buffer in the QEMUFile
with an offset buf_index, and there are arbitrary RAM pages
added typically via qemu_put_buffer_async.

The check for >= IO_BUF_SIZE is only done when adding to the f->buf,
where as the check on f->iovcnt is done when you add an element to
the iovec and that can happen potentially in either case.

Dave

> Assuming doing both flushs is ok for qemu_put_buffer_async
> then I'd suggest renaming 'add_to_iovec' to 'queue_buffer'
> and have that method do both of these qemu_fflush() calls.
> 
> > > ---
> > >  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
> > >  1 file changed, 28 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > > index 35c22605dd..05d9f42ddb 100644
> > > --- a/migration/qemu-file.c
> > > +++ b/migration/qemu-file.c
> > > @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
> > >      return ret;
> > >  }
> > >  
> > > -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > > -                         bool may_free)
> > > +/*
> > > + * Add buf to iovec. Do flush if iovec is full.
> > > + *
> > > + * Return values:
> > > + * 1 iovec is full and flushed
> > > + * 0 iovec is not flushed
> > > + *
> > > + */
> > > +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > > +                        bool may_free)
> > >  {
> > >      /* check for adjacent buffer and coalesce them */
> > >      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> > > @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > >  
> > >      if (f->iovcnt >= MAX_IOV_SIZE) {
> > >          qemu_fflush(f);
> > > +        return 1;
> > >      }
> > > +
> > > +    return 0;
> > >  }
> > >  
> > >  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> > > @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> > >          }
> > >          memcpy(f->buf + f->buf_index, buf, l);
> > >          f->bytes_xfer += l;
> > > -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> > > -        f->buf_index += l;
> > > -        if (f->buf_index == IO_BUF_SIZE) {
> > > -            qemu_fflush(f);
> > > +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
> > > +            f->buf_index += l;
> > > +            if (f->buf_index == IO_BUF_SIZE) {
> > > +                qemu_fflush(f);
> > > +            }
> > >          }
> > >          if (qemu_file_get_error(f)) {
> > >              break;
> > > @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
> > >  
> > >      f->buf[f->buf_index] = v;
> > >      f->bytes_xfer++;
> > > -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> > > -    f->buf_index++;
> > > -    if (f->buf_index == IO_BUF_SIZE) {
> > > -        qemu_fflush(f);
> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> > > +        f->buf_index++;
> > > +        if (f->buf_index == IO_BUF_SIZE) {
> > > +            qemu_fflush(f);
> > > +        }
> > >      }
> > >  }
> > >  
> > > @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
> > >      }
> > >  
> > >      qemu_put_be32(f, blen);
> > > -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> > > -    f->buf_index += blen;
> > > -    if (f->buf_index == IO_BUF_SIZE) {
> > > -        qemu_fflush(f);
> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
> > > +        f->buf_index += blen;
> > > +        if (f->buf_index == IO_BUF_SIZE) {
> > > +            qemu_fflush(f);
> > > +        }
> > >      }
> > >      return blen + sizeof(int32_t);
> > >  }
> > > -- 
> > > 2.17.1
> > > 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
> Regards,
> Daniel
> -- 
> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Wei Yang Aug. 24, 2019, 4:15 p.m. UTC | #4
On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
>(Copying Dan in)
>
>* Wei Yang (richardw.yang@linux.intel.com) wrote:
>> In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
>> this happens, buf_index is reset. Currently, this is not checked and
>> buf_index would always been adjust with buf size.
>> 
>> This is not harmful, but will waste some space in file buffer.
>
>That's a nice find.
>
>> This patch make add_to_iovec() return 1 when it has flushed the file.
>> Then the caller could check the return value to see whether it is
>> necessary to adjust the buf_index any more.
>> 
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>
>Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>
>(I wonder if there's a way to wrap that little add_to_iovec, check, add
>to index, flush in a little wrapper).
>
>Dave
>
>> ---
>>  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
>>  1 file changed, 28 insertions(+), 14 deletions(-)
>> 
>> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
>> index 35c22605dd..05d9f42ddb 100644
>> --- a/migration/qemu-file.c
>> +++ b/migration/qemu-file.c
>> @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
>>      return ret;
>>  }
>>  
>> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> -                         bool may_free)
>> +/*
>> + * Add buf to iovec. Do flush if iovec is full.
>> + *
>> + * Return values:
>> + * 1 iovec is full and flushed
>> + * 0 iovec is not flushed
>> + *
>> + */
>> +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> +                        bool may_free)
>>  {
>>      /* check for adjacent buffer and coalesce them */
>>      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
>> @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>>  
>>      if (f->iovcnt >= MAX_IOV_SIZE) {
>>          qemu_fflush(f);
>> +        return 1;
>>      }
>> +
>> +    return 0;
>>  }
>>  
>>  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
>> @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>>          }
>>          memcpy(f->buf + f->buf_index, buf, l);
>>          f->bytes_xfer += l;
>> -        add_to_iovec(f, f->buf + f->buf_index, l, false);
>> -        f->buf_index += l;
>> -        if (f->buf_index == IO_BUF_SIZE) {
>> -            qemu_fflush(f);
>> +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
>> +            f->buf_index += l;
>> +            if (f->buf_index == IO_BUF_SIZE) {
>> +                qemu_fflush(f);
>> +            }

You mean put these four lines into a wrapper?

Name it as add_buf_to_iovec?

>>          }
>>          if (qemu_file_get_error(f)) {
>>              break;
>> @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
>>  
>>      f->buf[f->buf_index] = v;
>>      f->bytes_xfer++;
>> -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
>> -    f->buf_index++;
>> -    if (f->buf_index == IO_BUF_SIZE) {
>> -        qemu_fflush(f);
>> +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
>> +        f->buf_index++;
>> +        if (f->buf_index == IO_BUF_SIZE) {
>> +            qemu_fflush(f);
>> +        }
>>      }
>>  }
>>  
>> @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>>      }
>>  
>>      qemu_put_be32(f, blen);
>> -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
>> -    f->buf_index += blen;
>> -    if (f->buf_index == IO_BUF_SIZE) {
>> -        qemu_fflush(f);
>> +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
>> +        f->buf_index += blen;
>> +        if (f->buf_index == IO_BUF_SIZE) {
>> +            qemu_fflush(f);
>> +        }
>>      }
>>      return blen + sizeof(int32_t);
>>  }
>> -- 
>> 2.17.1
>> 
>--
>Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Wei Yang Aug. 24, 2019, 4:22 p.m. UTC | #5
On Fri, Aug 23, 2019 at 02:05:02PM +0100, Dr. David Alan Gilbert wrote:
>* Daniel P. Berrang? (berrange@redhat.com) wrote:
>> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
>> > (Copying Dan in)
>> > 
>> > * Wei Yang (richardw.yang@linux.intel.com) wrote:
>> > > In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
>> > > this happens, buf_index is reset. Currently, this is not checked and
>> > > buf_index would always been adjust with buf size.
>> > > 
>> > > This is not harmful, but will waste some space in file buffer.
>> > 
>> > That's a nice find.
>> > 
>> > > This patch make add_to_iovec() return 1 when it has flushed the file.
>> > > Then the caller could check the return value to see whether it is
>> > > necessary to adjust the buf_index any more.
>> > > 
>> > > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> > 
>> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> > 
>> > (I wonder if there's a way to wrap that little add_to_iovec, check, add
>> > to index, flush in a little wrapper).
>> 
>> Given the name "add_to_iovec" I think it is pretty surprising
>> that it calls "qemu_flush" at all.
>> 
>> It is also pretty wierd that we're checking two different
>> conditions in two different places.
>> 
>> Right now the code is essentially doing this:
>> 
>>      if (f->iovcnt >= MAX_IOV_SIZE) {
>>         qemu_fflush(f);
>>      }
>>      if (f->buf_index == IO_BUF_SIZE) {
>>         qemu_fflush(f);
>>      }
>> 
>> Except that in the qemu_put_buffer_async() case, we're
>> only doing the first of these two checks. This feels
>> very odd indeed - I would have thought either it should
>> do both, or do neither.
>
>No, there's two separate types of buffers.
>
>There's f->buf which is a single allocated buffer in the QEMUFile
>with an offset buf_index, and there are arbitrary RAM pages
>added typically via qemu_put_buffer_async.
>

qemu_put_buffer_async is the only one which put a range not in f->buf into the
iovec.

And one thing confused me is even its name is async, add_to_iovec still would
call qemu_fflush when iovec is full. So it is not a always async function.
From the function name, it is a little difficult to differentiate
qemu_put_buffer and qemu_put_buffer_async.

>The check for >= IO_BUF_SIZE is only done when adding to the f->buf,
>where as the check on f->iovcnt is done when you add an element to
>the iovec and that can happen potentially in either case.
>
>Dave
>
>> Assuming doing both flushs is ok for qemu_put_buffer_async
>> then I'd suggest renaming 'add_to_iovec' to 'queue_buffer'
>> and have that method do both of these qemu_fflush() calls.
>> 
>> > > ---
>> > >  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
>> > >  1 file changed, 28 insertions(+), 14 deletions(-)
>> > > 
>> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
>> > > index 35c22605dd..05d9f42ddb 100644
>> > > --- a/migration/qemu-file.c
>> > > +++ b/migration/qemu-file.c
>> > > @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
>> > >      return ret;
>> > >  }
>> > >  
>> > > -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> > > -                         bool may_free)
>> > > +/*
>> > > + * Add buf to iovec. Do flush if iovec is full.
>> > > + *
>> > > + * Return values:
>> > > + * 1 iovec is full and flushed
>> > > + * 0 iovec is not flushed
>> > > + *
>> > > + */
>> > > +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> > > +                        bool may_free)
>> > >  {
>> > >      /* check for adjacent buffer and coalesce them */
>> > >      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
>> > > @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> > >  
>> > >      if (f->iovcnt >= MAX_IOV_SIZE) {
>> > >          qemu_fflush(f);
>> > > +        return 1;
>> > >      }
>> > > +
>> > > +    return 0;
>> > >  }
>> > >  
>> > >  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
>> > > @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>> > >          }
>> > >          memcpy(f->buf + f->buf_index, buf, l);
>> > >          f->bytes_xfer += l;
>> > > -        add_to_iovec(f, f->buf + f->buf_index, l, false);
>> > > -        f->buf_index += l;
>> > > -        if (f->buf_index == IO_BUF_SIZE) {
>> > > -            qemu_fflush(f);
>> > > +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
>> > > +            f->buf_index += l;
>> > > +            if (f->buf_index == IO_BUF_SIZE) {
>> > > +                qemu_fflush(f);
>> > > +            }
>> > >          }
>> > >          if (qemu_file_get_error(f)) {
>> > >              break;
>> > > @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
>> > >  
>> > >      f->buf[f->buf_index] = v;
>> > >      f->bytes_xfer++;
>> > > -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
>> > > -    f->buf_index++;
>> > > -    if (f->buf_index == IO_BUF_SIZE) {
>> > > -        qemu_fflush(f);
>> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
>> > > +        f->buf_index++;
>> > > +        if (f->buf_index == IO_BUF_SIZE) {
>> > > +            qemu_fflush(f);
>> > > +        }
>> > >      }
>> > >  }
>> > >  
>> > > @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>> > >      }
>> > >  
>> > >      qemu_put_be32(f, blen);
>> > > -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
>> > > -    f->buf_index += blen;
>> > > -    if (f->buf_index == IO_BUF_SIZE) {
>> > > -        qemu_fflush(f);
>> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
>> > > +        f->buf_index += blen;
>> > > +        if (f->buf_index == IO_BUF_SIZE) {
>> > > +            qemu_fflush(f);
>> > > +        }
>> > >      }
>> > >      return blen + sizeof(int32_t);
>> > >  }
>> > > -- 
>> > > 2.17.1
>> > > 
>> > --
>> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>> > 
>> 
>> Regards,
>> Daniel
>> -- 
>> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
>> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
>> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
>--
>Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Dr. David Alan Gilbert Sept. 3, 2019, 1:22 p.m. UTC | #6
* Wei Yang (richard.weiyang@gmail.com) wrote:
> On Fri, Aug 23, 2019 at 02:05:02PM +0100, Dr. David Alan Gilbert wrote:
> >* Daniel P. Berrang? (berrange@redhat.com) wrote:
> >> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
> >> > (Copying Dan in)
> >> > 
> >> > * Wei Yang (richardw.yang@linux.intel.com) wrote:
> >> > > In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> >> > > this happens, buf_index is reset. Currently, this is not checked and
> >> > > buf_index would always been adjust with buf size.
> >> > > 
> >> > > This is not harmful, but will waste some space in file buffer.
> >> > 
> >> > That's a nice find.
> >> > 
> >> > > This patch make add_to_iovec() return 1 when it has flushed the file.
> >> > > Then the caller could check the return value to see whether it is
> >> > > necessary to adjust the buf_index any more.
> >> > > 
> >> > > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> >> > 
> >> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >> > 
> >> > (I wonder if there's a way to wrap that little add_to_iovec, check, add
> >> > to index, flush in a little wrapper).
> >> 
> >> Given the name "add_to_iovec" I think it is pretty surprising
> >> that it calls "qemu_flush" at all.
> >> 
> >> It is also pretty wierd that we're checking two different
> >> conditions in two different places.
> >> 
> >> Right now the code is essentially doing this:
> >> 
> >>      if (f->iovcnt >= MAX_IOV_SIZE) {
> >>         qemu_fflush(f);
> >>      }
> >>      if (f->buf_index == IO_BUF_SIZE) {
> >>         qemu_fflush(f);
> >>      }
> >> 
> >> Except that in the qemu_put_buffer_async() case, we're
> >> only doing the first of these two checks. This feels
> >> very odd indeed - I would have thought either it should
> >> do both, or do neither.
> >
> >No, there's two separate types of buffers.
> >
> >There's f->buf which is a single allocated buffer in the QEMUFile
> >with an offset buf_index, and there are arbitrary RAM pages
> >added typically via qemu_put_buffer_async.
> >
> 
> qemu_put_buffer_async is the only one which put a range not in f->buf into the
> iovec.
> 
> And one thing confused me is even its name is async, add_to_iovec still would
> call qemu_fflush when iovec is full. So it is not a always async function.
> From the function name, it is a little difficult to differentiate
> qemu_put_buffer and qemu_put_buffer_async.

It's a bit confusing, but I think it is the right name;  the 'async'
refers to the fact that the data is read from the buffer we pass
sometime later, not synchronously before the return of the
qemu_put_buffer_async call.

Dave

> >The check for >= IO_BUF_SIZE is only done when adding to the f->buf,
> >where as the check on f->iovcnt is done when you add an element to
> >the iovec and that can happen potentially in either case.
> >
> >Dave
> >
> >> Assuming doing both flushs is ok for qemu_put_buffer_async
> >> then I'd suggest renaming 'add_to_iovec' to 'queue_buffer'
> >> and have that method do both of these qemu_fflush() calls.
> >> 
> >> > > ---
> >> > >  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
> >> > >  1 file changed, 28 insertions(+), 14 deletions(-)
> >> > > 
> >> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> >> > > index 35c22605dd..05d9f42ddb 100644
> >> > > --- a/migration/qemu-file.c
> >> > > +++ b/migration/qemu-file.c
> >> > > @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
> >> > >      return ret;
> >> > >  }
> >> > >  
> >> > > -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >> > > -                         bool may_free)
> >> > > +/*
> >> > > + * Add buf to iovec. Do flush if iovec is full.
> >> > > + *
> >> > > + * Return values:
> >> > > + * 1 iovec is full and flushed
> >> > > + * 0 iovec is not flushed
> >> > > + *
> >> > > + */
> >> > > +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >> > > +                        bool may_free)
> >> > >  {
> >> > >      /* check for adjacent buffer and coalesce them */
> >> > >      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> >> > > @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >> > >  
> >> > >      if (f->iovcnt >= MAX_IOV_SIZE) {
> >> > >          qemu_fflush(f);
> >> > > +        return 1;
> >> > >      }
> >> > > +
> >> > > +    return 0;
> >> > >  }
> >> > >  
> >> > >  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> >> > > @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> >> > >          }
> >> > >          memcpy(f->buf + f->buf_index, buf, l);
> >> > >          f->bytes_xfer += l;
> >> > > -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> >> > > -        f->buf_index += l;
> >> > > -        if (f->buf_index == IO_BUF_SIZE) {
> >> > > -            qemu_fflush(f);
> >> > > +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
> >> > > +            f->buf_index += l;
> >> > > +            if (f->buf_index == IO_BUF_SIZE) {
> >> > > +                qemu_fflush(f);
> >> > > +            }
> >> > >          }
> >> > >          if (qemu_file_get_error(f)) {
> >> > >              break;
> >> > > @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
> >> > >  
> >> > >      f->buf[f->buf_index] = v;
> >> > >      f->bytes_xfer++;
> >> > > -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> >> > > -    f->buf_index++;
> >> > > -    if (f->buf_index == IO_BUF_SIZE) {
> >> > > -        qemu_fflush(f);
> >> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> >> > > +        f->buf_index++;
> >> > > +        if (f->buf_index == IO_BUF_SIZE) {
> >> > > +            qemu_fflush(f);
> >> > > +        }
> >> > >      }
> >> > >  }
> >> > >  
> >> > > @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
> >> > >      }
> >> > >  
> >> > >      qemu_put_be32(f, blen);
> >> > > -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> >> > > -    f->buf_index += blen;
> >> > > -    if (f->buf_index == IO_BUF_SIZE) {
> >> > > -        qemu_fflush(f);
> >> > > +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
> >> > > +        f->buf_index += blen;
> >> > > +        if (f->buf_index == IO_BUF_SIZE) {
> >> > > +            qemu_fflush(f);
> >> > > +        }
> >> > >      }
> >> > >      return blen + sizeof(int32_t);
> >> > >  }
> >> > > -- 
> >> > > 2.17.1
> >> > > 
> >> > --
> >> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >> > 
> >> 
> >> Regards,
> >> Daniel
> >> -- 
> >> |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
> >> |: https://libvirt.org         -o-            https://fstop138.berrange.com :|
> >> |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
> >--
> >Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> -- 
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Dr. David Alan Gilbert Sept. 3, 2019, 6:43 p.m. UTC | #7
* Wei Yang (richard.weiyang@gmail.com) wrote:
> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
> >(Copying Dan in)
> >
> >* Wei Yang (richardw.yang@linux.intel.com) wrote:
> >> In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> >> this happens, buf_index is reset. Currently, this is not checked and
> >> buf_index would always been adjust with buf size.
> >> 
> >> This is not harmful, but will waste some space in file buffer.
> >
> >That's a nice find.
> >
> >> This patch make add_to_iovec() return 1 when it has flushed the file.
> >> Then the caller could check the return value to see whether it is
> >> necessary to adjust the buf_index any more.
> >> 
> >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> >
> >Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >
> >(I wonder if there's a way to wrap that little add_to_iovec, check, add
> >to index, flush in a little wrapper).
> >
> >Dave
> >
> >> ---
> >>  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
> >>  1 file changed, 28 insertions(+), 14 deletions(-)
> >> 
> >> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> >> index 35c22605dd..05d9f42ddb 100644
> >> --- a/migration/qemu-file.c
> >> +++ b/migration/qemu-file.c
> >> @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
> >>      return ret;
> >>  }
> >>  
> >> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >> -                         bool may_free)
> >> +/*
> >> + * Add buf to iovec. Do flush if iovec is full.
> >> + *
> >> + * Return values:
> >> + * 1 iovec is full and flushed
> >> + * 0 iovec is not flushed
> >> + *
> >> + */
> >> +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >> +                        bool may_free)
> >>  {
> >>      /* check for adjacent buffer and coalesce them */
> >>      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> >> @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> >>  
> >>      if (f->iovcnt >= MAX_IOV_SIZE) {
> >>          qemu_fflush(f);
> >> +        return 1;
> >>      }
> >> +
> >> +    return 0;
> >>  }
> >>  
> >>  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> >> @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> >>          }
> >>          memcpy(f->buf + f->buf_index, buf, l);
> >>          f->bytes_xfer += l;
> >> -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> >> -        f->buf_index += l;
> >> -        if (f->buf_index == IO_BUF_SIZE) {
> >> -            qemu_fflush(f);
> >> +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
> >> +            f->buf_index += l;
> >> +            if (f->buf_index == IO_BUF_SIZE) {
> >> +                qemu_fflush(f);
> >> +            }
> 
> You mean put these four lines into a wrapper?
> 
> Name it as add_buf_to_iovec?

Yes.

Dave

> >>          }
> >>          if (qemu_file_get_error(f)) {
> >>              break;
> >> @@ -412,10 +424,11 @@ void qemu_put_byte(QEMUFile *f, int v)
> >>  
> >>      f->buf[f->buf_index] = v;
> >>      f->bytes_xfer++;
> >> -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> >> -    f->buf_index++;
> >> -    if (f->buf_index == IO_BUF_SIZE) {
> >> -        qemu_fflush(f);
> >> +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> >> +        f->buf_index++;
> >> +        if (f->buf_index == IO_BUF_SIZE) {
> >> +            qemu_fflush(f);
> >> +        }
> >>      }
> >>  }
> >>  
> >> @@ -717,10 +730,11 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
> >>      }
> >>  
> >>      qemu_put_be32(f, blen);
> >> -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> >> -    f->buf_index += blen;
> >> -    if (f->buf_index == IO_BUF_SIZE) {
> >> -        qemu_fflush(f);
> >> +    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
> >> +        f->buf_index += blen;
> >> +        if (f->buf_index == IO_BUF_SIZE) {
> >> +            qemu_fflush(f);
> >> +        }
> >>      }
> >>      return blen + sizeof(int32_t);
> >>  }
> >> -- 
> >> 2.17.1
> >> 
> >--
> >Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> -- 
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
Wei Yang Sept. 11, 2019, 12:26 p.m. UTC | #8
On Tue, Sep 03, 2019 at 07:43:24PM +0100, Dr. David Alan Gilbert wrote:
>* Wei Yang (richard.weiyang@gmail.com) wrote:
>> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
>> >(Copying Dan in)
>> >
>> >* Wei Yang (richardw.yang@linux.intel.com) wrote:
>> >> In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
>> >> this happens, buf_index is reset. Currently, this is not checked and
>> >> buf_index would always been adjust with buf size.
>> >> 
>> >> This is not harmful, but will waste some space in file buffer.
>> >
>> >That's a nice find.
>> >
>> >> This patch make add_to_iovec() return 1 when it has flushed the file.
>> >> Then the caller could check the return value to see whether it is
>> >> necessary to adjust the buf_index any more.
>> >> 
>> >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> >
>> >Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> >
>> >(I wonder if there's a way to wrap that little add_to_iovec, check, add
>> >to index, flush in a little wrapper).
>> >
>> >Dave
>> >
>> >> ---
>> >>  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
>> >>  1 file changed, 28 insertions(+), 14 deletions(-)
>> >> 
>> >> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
>> >> index 35c22605dd..05d9f42ddb 100644
>> >> --- a/migration/qemu-file.c
>> >> +++ b/migration/qemu-file.c
>> >> @@ -343,8 +343,16 @@ int qemu_fclose(QEMUFile *f)
>> >>      return ret;
>> >>  }
>> >>  
>> >> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> >> -                         bool may_free)
>> >> +/*
>> >> + * Add buf to iovec. Do flush if iovec is full.
>> >> + *
>> >> + * Return values:
>> >> + * 1 iovec is full and flushed
>> >> + * 0 iovec is not flushed
>> >> + *
>> >> + */
>> >> +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> >> +                        bool may_free)
>> >>  {
>> >>      /* check for adjacent buffer and coalesce them */
>> >>      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
>> >> @@ -362,7 +370,10 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>> >>  
>> >>      if (f->iovcnt >= MAX_IOV_SIZE) {
>> >>          qemu_fflush(f);
>> >> +        return 1;
>> >>      }
>> >> +
>> >> +    return 0;
>> >>  }
>> >>  
>> >>  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
>> >> @@ -391,10 +402,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>> >>          }
>> >>          memcpy(f->buf + f->buf_index, buf, l);
>> >>          f->bytes_xfer += l;
>> >> -        add_to_iovec(f, f->buf + f->buf_index, l, false);
>> >> -        f->buf_index += l;
>> >> -        if (f->buf_index == IO_BUF_SIZE) {
>> >> -            qemu_fflush(f);
>> >> +        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
>> >> +            f->buf_index += l;
>> >> +            if (f->buf_index == IO_BUF_SIZE) {
>> >> +                qemu_fflush(f);
>> >> +            }
>> 
>> You mean put these four lines into a wrapper?
>> 
>> Name it as add_buf_to_iovec?
>
>Yes.

Sure, Let me prepare v2 with this.

Patch
diff mbox series

diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index 35c22605dd..05d9f42ddb 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -343,8 +343,16 @@  int qemu_fclose(QEMUFile *f)
     return ret;
 }
 
-static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
-                         bool may_free)
+/*
+ * Add buf to iovec. Do flush if iovec is full.
+ *
+ * Return values:
+ * 1 iovec is full and flushed
+ * 0 iovec is not flushed
+ *
+ */
+static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
+                        bool may_free)
 {
     /* check for adjacent buffer and coalesce them */
     if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
@@ -362,7 +370,10 @@  static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
 
     if (f->iovcnt >= MAX_IOV_SIZE) {
         qemu_fflush(f);
+        return 1;
     }
+
+    return 0;
 }
 
 void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
@@ -391,10 +402,11 @@  void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
         }
         memcpy(f->buf + f->buf_index, buf, l);
         f->bytes_xfer += l;
-        add_to_iovec(f, f->buf + f->buf_index, l, false);
-        f->buf_index += l;
-        if (f->buf_index == IO_BUF_SIZE) {
-            qemu_fflush(f);
+        if (!add_to_iovec(f, f->buf + f->buf_index, l, false)) {
+            f->buf_index += l;
+            if (f->buf_index == IO_BUF_SIZE) {
+                qemu_fflush(f);
+            }
         }
         if (qemu_file_get_error(f)) {
             break;
@@ -412,10 +424,11 @@  void qemu_put_byte(QEMUFile *f, int v)
 
     f->buf[f->buf_index] = v;
     f->bytes_xfer++;
-    add_to_iovec(f, f->buf + f->buf_index, 1, false);
-    f->buf_index++;
-    if (f->buf_index == IO_BUF_SIZE) {
-        qemu_fflush(f);
+    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
+        f->buf_index++;
+        if (f->buf_index == IO_BUF_SIZE) {
+            qemu_fflush(f);
+        }
     }
 }
 
@@ -717,10 +730,11 @@  ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
     }
 
     qemu_put_be32(f, blen);
-    add_to_iovec(f, f->buf + f->buf_index, blen, false);
-    f->buf_index += blen;
-    if (f->buf_index == IO_BUF_SIZE) {
-        qemu_fflush(f);
+    if (!add_to_iovec(f, f->buf + f->buf_index, blen, false)) {
+        f->buf_index += blen;
+        if (f->buf_index == IO_BUF_SIZE) {
+            qemu_fflush(f);
+        }
     }
     return blen + sizeof(int32_t);
 }