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

[Kevin Wolf <kwolf@redhat.com>]

Am 04.07.2018 um 18:11 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 04.07.2018 18:08, Kevin Wolf wrote:
> > 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.
> 
> But in case of aligned request, there no previously read data, and we can
> safely continue. And actually it's our case (backup writes are aligned).

Hm, right. I don't particularly like req->partial because it's easy to
forget to set it to false when you do something that would need to be
repeated, but I don't have a better idea.

Kevin