Re: [Qemu-devel] [PATCH 2/2] migration/qemu-file: fix potential buf waste for extra buf_index adjustment

["Daniel P. Berrangé" <berrange@redhat.com>]

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
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