* Wei Yang (richard.weiy...@gmail.com) wrote:
> On Fri, Aug 23, 2019 at 02:05:02PM +0100, Dr. David Alan Gilbert wrote:
> >* Daniel P. Berrang? (berra...@redhat.com) wrote:
> >> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
> >> > (Copying Dan in)
> >> >
> >> > * Wei Yang (richardw.y...@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.y...@linux.intel.com>
> >> >
> >> > Reviewed-by: Dr. David Alan Gilbert <dgilb...@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 / dgilb...@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 / dgilb...@redhat.com / Manchester, UK
>
> --
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
