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.
>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
