Re: [PATCH 1/4] block/file-posix: set up Linux AIO and io_uring in the current thread

[Eric Blake]

On Thu, Sep 14, 2023 at 10:00:58AM -0400, Stefan Hajnoczi wrote:
> The file-posix block driver currently only sets up Linux AIO and
> io_uring in the BDS's AioContext. In the multi-queue block layer we must
> be able to submit I/O requests in AioContexts that do not have Linux AIO
> and io_uring set up yet since any thread can call into the block driver.
> 
> Set up Linux AIO and io_uring for the current AioContext during request
> submission. We lose the ability to return an error from
> .bdrv_file_open() when Linux AIO and io_uring setup fails (e.g. due to
> resource limits). Instead the user only gets warnings and we fall back
> to aio=threads. This is still better than a fatal error after startup.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/file-posix.c | 99 ++++++++++++++++++++++------------------------
>  1 file changed, 47 insertions(+), 52 deletions(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 4757914ac0..e9dbb87c57 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -713,17 +713,11 @@ static int raw_open_common(BlockDriverState *bs, QDict 
> *options,
>  
>  #ifdef CONFIG_LINUX_AIO
>       /* Currently Linux does AIO only for files opened with O_DIRECT */
> -    if (s->use_linux_aio) {
> -        if (!(s->open_flags & O_DIRECT)) {
> -            error_setg(errp, "aio=native was specified, but it requires "
> -                             "cache.direct=on, which was not specified.");
> -            ret = -EINVAL;
> -            goto fail;
> -        }
> -        if (!aio_setup_linux_aio(bdrv_get_aio_context(bs), errp)) {

We were previously doing setup only once during open...

> +static inline bool raw_check_linux_io_uring(BDRVRawState *s)
> +{
> +    Error *local_err = NULL;
> +    AioContext *ctx;
> +
> +    if (!s->use_linux_io_uring) {
> +        return false;
> +    }
> +
> +    ctx = qemu_get_current_aio_context();
> +    if (unlikely(!aio_setup_linux_io_uring(ctx, &local_err))) {

...now you're doing it on ever I/O request.  I had to check that setup
is idempotent; thankfully it is (once ctx has an associated linux_aio
or io_uring, setup is a no-op for subsequent calls).

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org