Re: [PATCH v3] linux-aio: add IO_CMD_FDSYNC command support

[Stefan Hajnoczi]

On Wed, Mar 13, 2024 at 02:19:35PM +0530, Prasad Pandit wrote:
> From: Prasad Pandit <pjp@fedoraproject.org>
> 
> Libaio defines IO_CMD_FDSYNC command to sync all outstanding
> asynchronous I/O operations, by flushing out file data to the
> disk storage.
> 
> Enable linux-aio to submit such aio request. This helps to
> reduce latency induced via pthread_create calls by
> thread-pool (aio=threads).
> 
> Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
> ---
>  block/file-posix.c |  7 +++++++
>  block/linux-aio.c  | 22 +++++++++++++++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
> 
> v3: check if host kernel supports aio_fsync call
>   -> https://lists.nongnu.org/archive/html/qemu-devel/2024-03/msg03210.html
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 35684f7e21..a30494d1a8 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -2453,6 +2453,8 @@ static inline bool 
> raw_check_linux_io_uring(BDRVRawState *s)
>  #endif
>  
>  #ifdef CONFIG_LINUX_AIO
> +extern bool laio_has_fdsync(int);

Please declare this in include/block/raw-aio.h alongside the other laio APIs.

> +
>  static inline bool raw_check_linux_aio(BDRVRawState *s)
>  {
>      Error *local_err = NULL;
> @@ -2599,6 +2601,11 @@ static int coroutine_fn 
> raw_co_flush_to_disk(BlockDriverState *bs)
>      if (raw_check_linux_io_uring(s)) {
>          return luring_co_submit(bs, s->fd, 0, NULL, QEMU_AIO_FLUSH);
>      }
> +#endif
> +#ifdef CONFIG_LINUX_AIO
> +    if (raw_check_linux_aio(s) && laio_has_fdsync(s->fd)) {

FDSYNC support should be probed at open() time and the result should be
stored in a new bool field like s->laio_supports_fdsync. That way the
cost of laio_has_fdsync() on every flush request is avoided.

> +        return laio_co_submit(s->fd, 0, NULL, QEMU_AIO_FLUSH, 0);
> +    }
>  #endif
>      return raw_thread_pool_submit(handle_aiocb_flush, &acb);
>  }
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index ec05d946f3..ed20a503e9 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -67,6 +67,7 @@ struct LinuxAioState {
>      int event_max;
>  };
>  
> +bool laio_has_fdsync(int);
>  static void ioq_submit(LinuxAioState *s);
>  
>  static inline ssize_t io_event_ret(struct io_event *ev)
> @@ -384,6 +385,9 @@ static int laio_do_submit(int fd, struct qemu_laiocb 
> *laiocb, off_t offset,
>      case QEMU_AIO_READ:
>          io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
>          break;
> +    case QEMU_AIO_FLUSH:
> +        io_prep_fdsync(iocbs, fd);
> +        break;
>      /* Currently Linux kernel does not support other operations */
>      default:
>          fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
> @@ -412,7 +416,7 @@ int coroutine_fn laio_co_submit(int fd, uint64_t offset, 
> QEMUIOVector *qiov,
>      AioContext *ctx = qemu_get_current_aio_context();
>      struct qemu_laiocb laiocb = {
>          .co         = qemu_coroutine_self(),
> -        .nbytes     = qiov->size,
> +        .nbytes     = qiov ? qiov->size : 0,
>          .ctx        = aio_get_linux_aio(ctx),
>          .ret        = -EINPROGRESS,
>          .is_read    = (type == QEMU_AIO_READ),
> @@ -486,3 +490,19 @@ void laio_cleanup(LinuxAioState *s)
>      }
>      g_free(s);
>  }
> +
> +bool laio_has_fdsync(int fd)
> +{
> +    struct iocb cb;
> +    struct iocb *cbs[] = {&cb, NULL};
> +
> +    io_context_t ctx = 0;
> +    io_setup(1, &ctx);
> +
> +    /* check if host kernel supports IO_CMD_FDSYNC */
> +    io_prep_fdsync(&cb, fd);
> +    int ret = io_submit(ctx, 1, cbs);
> +
> +    io_destroy(ctx);
> +    return (ret == -EINVAL) ? false : true;
> +}
> -- 
> 2.44.0
>

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