Re: [Qemu-block] [PATCH v4] linux-aio: properly bubble up errors from initialization

[Stefan Hajnoczi]

On Fri, Jun 22, 2018 at 12:37:00PM -0700, Nishanth Aravamudan wrote:
> laio_init() can fail for a couple of reasons, which will lead to a NULL
> pointer dereference in laio_attach_aio_context().
> 
> To solve this, add a aio_setup_linux_aio() function which is called
> early in raw_open_common. If this fails, propagate the error up. The
> signature of aio_get_linux_aio() was not modified, because it seems
> preferable to return the actual errno from the possible failing
> initialization calls.
> 
> Additionally, when the AioContext changes, we need to associate a
> LinuxAioState with the new AioContext. Use the bdrv_attach_aio_context
> callback and call the new aio_setup_linux_aio(), which will allocate a
> new AioContext if needed, and return errors on failures. If it fails for
> any reason, fallback to threaded AIO with an error message, as the
> device is already in-use by the guest.
> 
> Add an assert that aio_get_linux_aio() cannot return NULL.
> 
> Signed-off-by: Nishanth Aravamudan <address@hidden>
> 
> ---
> 
> Test case 1:
>     Set /proc/sys/fs/max-aio-nr to 0. Start a guest with an aio=native
>     disk.
> 
>     Result: laio_init() returns NULL due to not being able to allocate
>     any AIO contexts. This NULL is assigned to ctx->linux_aio and
>     dereferenced in aio_get_linux_aio.
> 
> Test case 2:
>     Set /proc/sys/fs/max-aio-nr to 128. Start a guest with an aio=native
>     disk and one in-use I/O thread.
> 
>     Result: laio_init() returns NULL due to not being able to allocate
>     additional AIO contexts for the I/O thread. This NULL is assigned to
>     ctx->linux_aio and dereferenced in aio_get_linux_aio.
> 
> Changes from v3 -> v4 (thanks to Fam Zheng and Kevin Wolf for review):
> 
> Squash patch 2/2 into 1/2.
> Differentiate the possible reasons for laio_init() failing and use an
> Error ** to propagate the errno to the caller.
> 
> Changes from v2 -> v3 (thanks to Eric Blake and Kevin Wolf for review):
> 
> Use a boolean false rather than 0 in assignment to use_linux_aio.
> Drop ending '.' from error_report() calls.
> Fix typo in commit message (propogates -> propagates).
> Move aio_setup_linux_aio call to raw_open_common.
> 
> Changes from v1 -> v2 (thanks to Kevin Wolf for review):
> 
> Rather than affect virtio-scsi/blk at all, make all the changes internal
> to file-posix.c. Thanks to Kevin Wolf for the suggested change.
> 
>  block/file-posix.c      | 33 ++++++++++++++++++++++++++++-----
>  block/linux-aio.c       | 12 +++++++++---
>  include/block/aio.h     |  3 +++
>  include/block/raw-aio.h |  2 +-
>  stubs/linux-aio.c       |  2 +-
>  util/async.c            | 14 +++++++++++---
>  6 files changed, 53 insertions(+), 13 deletions(-)

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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