Re: [Qemu-block] [PATCH V2 0/4] ide: avoid main-loop hang on CDROM/NFS failure

[Peter Lieven]

Am 26.10.2015 um 11:42 schrieb Stefan Hajnoczi:
> On Mon, Oct 12, 2015 at 02:27:21PM +0200, Peter Lieven wrote:
> > This series aims at avoiding a hanging main-loop if a vserver has a
> > CDROM image mounted from a NFS share and that NFS share goes down.
> > Typical situation is that users mount an CDROM ISO to install something
> > and then forget to eject that CDROM afterwards.
> > As a consequence this mounted CD is able to bring down the
> > whole vserver if the backend NFS share is unreachable. This is bad
> > especially if the CDROM itself is not needed anymore at this point.
> >
> > This series aims at fixing 2 blocking I/O operations that would
> > hang if the NFS server is unavailable:
> >   - ATAPI PIO read requests used sync calls to blk_read, convert
> >     them to an async variant where possible.
> >   - If a busmaster DMA request is cancelled all requests are drained.
> >     Convert the drain to an async request canceling.
> >
> > v1->v2: - fix offset for 2352 byte sector size [Kevin]
> >          - use a sync request if we continue an elementary transfer.
> >            As John pointed out we enter a race condition between next
> >            IDE command and async transfer otherwise. This is sill not
> >            optimal, but it fixes the NFS down problems for all cases where
> >            the NFS server goes down while there is no PIO CD activity.
> >            Of course, it could still happen during a PIO transfer, but I
> >            expect this to be the unlikelier case.
> >            I spent some effort trying to read more sectors at once and
> >            avoiding continuation of elementary transfers, but with
> >            whatever I came up it was destroying migration between different
> >            Qemu versions. I have a quite hackish patch that works and
> >            should survive migration, but I am not happy with it. So I
> >            would like to start with this version as it is a big improvement
> >            already.
> >          - Dropped Patch 5 because it is upstream meanwhile.
> >
> > Peter Lieven (4):
> >    ide/atapi: make PIO read requests async
> >    ide/atapi: blk_aio_readv may return NULL
> >    ide: add support for cancelable read requests
> >    ide/atapi: enable cancelable requests
> >
> >   hw/ide/atapi.c    | 99 +++++++++++++++++++++++++++++++++++++++++++++++++------
> >   hw/ide/core.c     | 55 +++++++++++++++++++++++++++++++
> >   hw/ide/internal.h | 16 +++++++++
> >   hw/ide/pci.c      | 42 +++++++++++++++--------
> >   4 files changed, 188 insertions(+), 24 deletions(-)
> Any reason why write and discard requests aren't covered in this series?
>
> If this is a good idea for CD-ROM it should be a good idea for all PCI
> IDE devices.
>
> Having a specialized code path is often a sign that it hasn't been
> tested enough.  Can we get confident enough to enable this everywhere?

The reason is that the buffered request trick does only work for
read-only devices (like a CDROM). A write request that is completed
on the backend storage at a later point (after the OS thinks the request
is canceled) can cause damage to the filesystem.

Peter