io_uring: expose single mmap capability

After commit 75b28affdd6a we can get by with just a single mmap to
map both the sq and cq ring. However, userspace doesn't know that.

Add a features variable to io_uring_params, and notify userspace
that the kernel has this ability. This can then be used in liburing
(or in applications directly) to avoid the second mmap.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 17dfe57..be24596 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3391,6 +3391,8 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p)
        p->cq_off.ring_entries = offsetof(struct io_rings, cq_ring_entries);
        p->cq_off.overflow = offsetof(struct io_rings, cq_overflow);
        p->cq_off.cqes = offsetof(struct io_rings, cqes);
+
+       p->features = IORING_FEAT_SINGLE_MMAP;
        return ret;
 err:
        io_ring_ctx_wait_and_kill(ctx);

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 1e1652f..96ee9d9 100644 (file)
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -128,12 +128,18 @@ struct io_uring_params {
        __u32 flags;
        __u32 sq_thread_cpu;
        __u32 sq_thread_idle;
-       __u32 resv[5];
+       __u32 features;
+       __u32 resv[4];
        struct io_sqring_offsets sq_off;
        struct io_cqring_offsets cq_off;
 };
 
 /*
+ * io_uring_params->features flags
+ */
+#define IORING_FEAT_SINGLE_MMAP                (1U << 0)
+
+/*
  * io_uring_register(2) opcodes and arguments
  */
 #define IORING_REGISTER_BUFFERS                0