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

[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?