Re: [Qemu-devel] [PATCH v7 4/9] block: treat BDRV_REQ_ALLOCATE as serialising

[Anton Nefedov]

On 29/1/2018 10:48 PM, Max Reitz wrote:
> On 2018-01-18 18:49, Anton Nefedov wrote:
> > The idea is that ALLOCATE requests may overlap with other requests.
> > Reuse the existing block layer infrastructure for serialising requests.
> > Use the following approach:
> >    - mark ALLOCATE serialising, so subsequent requests to the area wait
> >    - ALLOCATE request itself must never wait if another request is in flight
> >      already. Return EAGAIN, let the caller reconsider.
> >
> > Signed-off-by: Anton Nefedov <address@hidden>
> > Reviewed-by: Eric Blake <address@hidden>
> > ---
> >   block/io.c | 27 +++++++++++++++++++--------
> >   1 file changed, 19 insertions(+), 8 deletions(-)
>
> The basic principle looks good to me.
>
> > diff --git a/block/io.c b/block/io.c
> > index cf2f84c..4b0d34f 100644
> > --- a/block/io.c
> > +++ b/block/io.c
>
> [...]
>
> > @@ -1717,7 +1728,7 @@ int coroutine_fn bdrv_co_pwritev(BdrvChild *child,
> >           struct iovec head_iov;
> >   
> >           mark_request_serialising(&req, align);
> > -        wait_serialising_requests(&req);
> > +        wait_serialising_requests(&req, false);
>
> What if someone calls bdrv_co_pwritev() with BDRV_REQ_ZERO_WRITE |
> BDRV_REQ_ALLOCATE?  

Either

    assert(!(qiov && (flags & BDRV_REQ_ALLOCATE)));

will fail or bdrv_co_do_zero_pwritev() will be used.

> .. Then this should do exactly the same as
> bdrv_co_do_zero_pwritev(), which it currently does not -- besides this
> serialization, this includes returning -ENOTSUP if there is a head or
> tail to write.

Another question is if that assertion is ok.
In other words: should (qiov!=NULL && REQ_ALLOCATE) be a valid case?
e.g. with qiov filled with zeroes?

I'd rather document that not supported (and leave the assertion).

Actually, even (qiov!=NULL && REQ_ZERO_WRITE) looks kind of
unsupported/broken? Alignment code in bdrv_co_pwritev() zeroes out the
head and tail by passing the flag down bdrv_aligned_pwritev()