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

[Wei Yang]

On Fri, Aug 23, 2019 at 02:05:02PM +0100, Dr. David Alan Gilbert wrote:
>* Daniel P. Berrang? (address@hidden) wrote:
>> On Fri, Aug 23, 2019 at 12:06:09PM +0100, Dr. David Alan Gilbert wrote:
>> > (Copying Dan in)
>> > 
>> > * Wei Yang (address@hidden) 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 <address@hidden>
>> > 
>> > Reviewed-by: Dr. David Alan Gilbert <address@hidden>
>> > 
>> > (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 / address@hidden / Manchester, UK
>> > 
>> 
>> Regards,
>> Daniel
>> -- 
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>--
>Dr. David Alan Gilbert / address@hidden / Manchester, UK

-- 
Wei Yang
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