Re: [Qemu-devel] [PATCH v6 2/2] block: Support GlusterFS as a QEMU block backend

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Thu, Sep 06, 2012 at 09:35:04AM +0200, Paolo Bonzini wrote:
> > +static int qemu_gluster_open(BlockDriverState *bs, const char *filename,
> > +    int bdrv_flags)
> > +{
> > +    BDRVGlusterState *s = bs->opaque;
> > +    int open_flags = 0;
> > +    int ret = 0;
> > +    GlusterURI *uri = g_malloc0(sizeof(GlusterURI));
> > +
> > +    s->glfs = qemu_gluster_init(uri, filename);
> > +    if (!s->glfs) {
> > +        ret = -errno;
> > +        goto out;
> > +    }
> > +
> > +    open_flags |=  O_BINARY;
> > +    open_flags &= ~O_ACCMODE;
> > +    if (bdrv_flags & BDRV_O_RDWR) {
> > +        open_flags |= O_RDWR;
> > +    } else {
> > +        open_flags |= O_RDONLY;
> > +    }
> > +
> > +    if ((bdrv_flags & BDRV_O_NOCACHE)) {
> > +        open_flags |= O_DIRECT;
> > +    }
> > +
> > +    s->fd = glfs_open(s->glfs, uri->image, open_flags);
> > +    if (!s->fd) {
> > +        ret = -errno;
> > +        goto out;
> > +    }
> > +
> > +    ret = qemu_pipe(s->fds);
> > +    if (ret < 0) {
> > +        goto out;
> > +    }
> > +    fcntl(s->fds[0], F_SETFL, O_NONBLOCK);
> > +    fcntl(s->fds[1], F_SETFL, O_NONBLOCK);
> 
> A small thing I noticed while reviewing: since the write end of the pipe
> is used from the gluster thread, you do not need to make this nonblocking.

Ok.

> 
> Also, please use GLUSTER_FD_{READ,WRITE} instead.

Sure.

> > +static void qemu_gluster_aio_cancel(BlockDriverAIOCB *blockacb)
> > +{
> > +    GlusterAIOCB *acb = (GlusterAIOCB *)blockacb;
> > +
> > +    acb->common.cb(acb->common.opaque, -ECANCELED);
> > +    acb->canceled = true;
> 
> I think this is wrong, because the write could still complete later and
> undo the effects of a second write that is done by the guest.  That is:
> 
>    gluster                  QEMU                      guest
> ----------------------------------------------------------
>                                              <---     write #1
>                      <---   write #1
>                                              <---     cancel write #1
>                             write #1 canceled --->
>                                              <---     write #2
>                      <---   write #2
>    write #2 completed --->
>                             write #2 completed -->
>    write #1 completed --->
> 
> Now, the persistent storage recorded the effect of write #1, but the
> guest thinks that it recorded the effect of write #2 instead.
> 
> You can simply do qemu_aio_flush() here.

Based on my discussions with Kevin, I have now followed block/qed.c for
aio_cancel which is based on using acb->finished.

> > +static int qemu_gluster_send_pipe(BDRVGlusterState *s, GlusterAIOCB *acb)
> > +{
> > +    int ret = 0;
> > +    while (1) {
> > +        fd_set wfd;
> > +        int fd = s->fds[GLUSTER_FD_WRITE];
> > +
> > +        ret = write(fd, (void *)&acb, sizeof(acb));
> > +        if (ret >= 0) {
> > +            break;
> > +        }
> > +        if (errno == EINTR) {
> > +            continue;
> > +        }
> > +        if (errno != EAGAIN) {
> > +            break;
> > +        }
> > +
> > +        FD_ZERO(&wfd);
> > +        FD_SET(fd, &wfd);
> > +        do {
> > +            ret = select(fd + 1, NULL, &wfd, NULL, NULL);
> > +        } while (ret < 0 && errno == EINTR);
> 
> If you make the fd non-blocking, you can avoid the select here.

Will change in in the next iteration.

Thanks for taking time to review.

Regards,
Bharata.