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 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).
>
>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 / address@hidden / Manchester, UK

-- 
Wei Yang
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