Re: [Qemu-devel] [PATCH 1/2] block: add BDRV_REQ_SERIALISING flag

[Kevin Wolf <kwolf@redhat.com>]

Am 04.07.2018 um 16:44 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 03.07.2018 21:07, Vladimir Sementsov-Ogievskiy wrote:
> > Serialized writes should be used in copy-on-write of backup(sync=none)
> > for image fleecing scheme.
> > 
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > ---
> >   include/block/block.h | 5 ++++-
> >   block/io.c            | 4 ++++
> >   2 files changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/block/block.h b/include/block/block.h
> > index e5c7759a0c..107113aad5 100644
> > --- a/include/block/block.h
> > +++ b/include/block/block.h
> > @@ -58,8 +58,11 @@ typedef enum {
> >        * content. */
> >       BDRV_REQ_WRITE_UNCHANGED    = 0x40,
> > +    /* Force request serializing. Only for writes. */
> > +    BDRV_REQ_SERIALISING        = 0x80,
> > +
> >       /* Mask of valid flags */
> > -    BDRV_REQ_MASK               = 0x7f,
> > +    BDRV_REQ_MASK               = 0xff,
> >   } BdrvRequestFlags;
> >   typedef struct BlockSizes {
> > diff --git a/block/io.c b/block/io.c
> > index 1a2272fad3..d5ba078514 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -1572,6 +1572,10 @@ static int coroutine_fn bdrv_aligned_pwritev(BdrvChild *child,
> >       max_transfer = QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_transfer, INT_MAX),
> >                                      align);
> > +    if (flags & BDRV_REQ_SERIALISING) {
> > +        mark_request_serialising(req, bdrv_get_cluster_size(bs));
> > +    }
> > +
> >       waited = wait_serialising_requests(req);
> >       assert(!waited || !req->serialising);
> 
> Kevin, about this assertion, introduced in 28de2dcd88de "block: Assert
> serialisation assumptions in pwritev"? Will not it fail with fleecing
> scheme? I'm afraid it will, when we will wait for client read with our
> request, marked serializing a moment ago...

Hm, looks like it yes.

> Can we just switch it to assert(!waited || !req->partial);, setting
> req->partial in bdrv_co_pwritev for parts of unaligned requests? And allow
> new flag only for aligned requests?
> 
> Other ideas?

The commit message of 28de2dcd88de tells you what we need to do (and
that just changing the assertion is wrong):

    If a request calls wait_serialising_requests() and actually has to wait
    in this function (i.e. a coroutine yield), other requests can run and
    previously read data (like the head or tail buffer) could become
    outdated. In this case, we would have to restart from the beginning to
    read in the updated data.

    However, we're lucky and don't actually need to do that: A request can
    only wait in the first call of wait_serialising_requests() because we
    mark it as serialising before that call, so any later requests would
    wait. So as we don't wait in practice, we don't have to reload the data.

    This is an important assumption that may not be broken or data
    corruption will happen. Document it with some assertions.

So we may need to return -EAGAIN here, check that in the caller and
repeat the write request from the very start.

Kevin