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

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Sep 03, 2019 at 07:43:24PM +0100, Dr. David Alan Gilbert wrote:
>* Wei Yang (richard.weiyang@gmail.com) wrote:
>> 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).
>> >
>> >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?
>
>Yes.

Sure, Let me prepare v2 with this.


-- 
Wei Yang
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