[PATCH 3/6] io_uring: add support for kernel registered bvecs

Keith Busch
2025-02-03T15:45:30Z

From: Keith Busch <kbusch@kernel.org>

Provide an interface for the kernel to leverage the existing
pre-registered buffers that io_uring provides. User space can reference
these later to achieve zero-copy IO.

User space must register an empty fixed buffer table with io_uring in
order for the kernel to make use of it.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
include/linux/io_uring.h | 1 +
include/linux/io_uring_types.h | 3 +
io_uring/rsrc.c | 114 +++++++++++++++++++++++++++++++--
io_uring/rsrc.h | 1 +
4 files changed, 114 insertions(+), 5 deletions(-)

diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
index 85fe4e6b275c7..b5637a2aae340 100644
--- a/include/linux/io_uring.h
+++ b/include/linux/io_uring.h
@@ -5,6 +5,7 @@
#include <linux/sched.h>
#include <linux/xarray.h>
#include <uapi/linux/io_uring.h>
+#include <linux/blk-mq.h>

#if defined(CONFIG_IO_URING)
void __io_uring_cancel(bool cancel_all);
diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 623d8e798a11a..7e5a5a70c35f2 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -695,4 +695,7 @@ static inline bool io_ctx_cqe32(struct io_ring_ctx *ctx)
return ctx->flags & IORING_SETUP_CQE32;
}

+int io_buffer_register_bvec(struct io_ring_ctx *ctx, const struct request *rq, unsigned int tag);
+void io_buffer_unregister_bvec(struct io_ring_ctx *ctx, unsigned int tag);
+
#endif
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 4d0e1c06c8bc6..8c4c374abcc10 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -111,7 +111,10 @@ static void io_buffer_unmap(struct io_ring_ctx *ctx, struct io_rsrc_node *node)
if (!refcount_dec_and_test(&imu->refs))
return;
for (i = 0; i < imu->nr_bvecs; i++)
- unpin_user_page(imu->bvec[i].bv_page);
+ if (node->type == IORING_RSRC_KBUF)
+ put_page(imu->bvec[i].bv_page);
+ else
+ unpin_user_page(imu->bvec[i].bv_page);
if (imu->acct_pages)
io_unaccount_mem(ctx, imu->acct_pages);
kvfree(imu);
@@ -240,6 +243,13 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
struct io_rsrc_node *node;
u64 tag = 0;

+ i = array_index_nospec(up->offset + done, ctx->buf_table.nr);
+ node = io_rsrc_node_lookup(&ctx->buf_table, i);
+ if (node && node->type != IORING_RSRC_BUFFER) {
+ err = -EBUSY;
+ break;
+ }
+
uvec = u64_to_user_ptr(user_data);
iov = iovec_from_user(uvec, 1, 1, &fast_iov, ctx->compat);
if (IS_ERR(iov)) {
@@ -258,6 +268,7 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
err = PTR_ERR(node);
break;
}
+
if (tag) {
if (!node) {
err = -EINVAL;
@@ -265,7 +276,6 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
}
node->tag = tag;
}
- i = array_index_nospec(up->offset + done, ctx->buf_table.nr);
io_reset_rsrc_node(ctx, &ctx->buf_table, i);
ctx->buf_table.nodes[i] = node;
if (ctx->compat)
@@ -453,6 +463,7 @@ void io_free_rsrc_node(struct io_ring_ctx *ctx, struct io_rsrc_node *node)
fput(io_slot_file(node));
break;
case IORING_RSRC_BUFFER:
+ case IORING_RSRC_KBUF:
if (node->buf)
io_buffer_unmap(ctx, node);
break;
@@ -860,6 +871,92 @@ int io_sqe_buffers_register(struct io_ring_ctx *ctx, void __user *arg,
return ret;
}

+static struct io_rsrc_node *io_buffer_alloc_node(struct io_ring_ctx *ctx,
+ unsigned int nr_bvecs,
+ unsigned int len)
+{
+ struct io_mapped_ubuf *imu;
+ struct io_rsrc_node *node;
+
+ node = io_rsrc_node_alloc(IORING_RSRC_KBUF);
+ if (!node)
+ return NULL;
+
+ imu = kvmalloc(struct_size(imu, bvec, nr_bvecs), GFP_KERNEL);
+ if (!imu) {
+ io_put_rsrc_node(ctx, node);
+ return NULL;
+ }
+
+ imu->ubuf = 0;
+ imu->len = len;
+ imu->acct_pages = 0;
+ imu->nr_bvecs = nr_bvecs;
+ refcount_set(&imu->refs, 1);
+
+ node->buf = imu;
+ return node;
+}
+
+int io_buffer_register_bvec(struct io_ring_ctx *ctx, const struct request *rq,
+ unsigned int index)
+{
+ struct io_rsrc_data *data = &ctx->buf_table;
+ u16 nr_bvecs = blk_rq_nr_phys_segments(rq);
+ struct req_iterator rq_iter;
+ struct io_rsrc_node *node;
+ struct bio_vec bv;
+ int i = 0;
+
+ lockdep_assert_held(&ctx->uring_lock);
+
+ if (WARN_ON_ONCE(!data->nr))
+ return -EINVAL;
+ if (WARN_ON_ONCE(index >= data->nr))
+ return -EINVAL;
+
+ node = data->nodes[index];
+ if (WARN_ON_ONCE(node))
+ return -EBUSY;
+
+ node = io_buffer_alloc_node(ctx, nr_bvecs, blk_rq_bytes(rq));
+ if (!node)
+ return -ENOMEM;
+
+ rq_for_each_bvec(bv, rq, rq_iter) {
+ get_page(bv.bv_page);
+ node->buf->bvec[i].bv_page = bv.bv_page;
+ node->buf->bvec[i].bv_len = bv.bv_len;
+ node->buf->bvec[i].bv_offset = bv.bv_offset;
+ i++;
+ }
+ data->nodes[index] = node;
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(io_buffer_register_bvec);
+
+void io_buffer_unregister_bvec(struct io_ring_ctx *ctx, unsigned int index)
+{
+ struct io_rsrc_data *data = &ctx->buf_table;
+ struct io_rsrc_node *node;
+
+ lockdep_assert_held(&ctx->uring_lock);
+
+ if (WARN_ON_ONCE(!data->nr))
+ return;
+ if (WARN_ON_ONCE(index >= data->nr))
+ return;
+
+ node = data->nodes[index];
+ if (WARN_ON_ONCE(!node || !node->buf))
+ return;
+ if (WARN_ON_ONCE(node->type != IORING_RSRC_KBUF))
+ return;
+ io_reset_rsrc_node(ctx, data, index);
+}
+EXPORT_SYMBOL_GPL(io_buffer_unregister_bvec);
+
int io_import_fixed(int ddir, struct iov_iter *iter, struct io_rsrc_node *node,
u64 buf_addr, size_t len)
{
@@ -886,8 +983,8 @@ int io_import_fixed(int ddir, struct iov_iter *iter, struct io_rsrc_node *node,
/*
* Don't use iov_iter_advance() here, as it's really slow for
* using the latter parts of a big fixed buffer - it iterates
- * over each segment manually. We can cheat a bit here, because
- * we know that:
+ * over each segment manually. We can cheat a bit here for user
+ * registered nodes, because we know that:
*
* 1) it's a BVEC iter, we set it up
* 2) all bvecs are the same in size, except potentially the
@@ -901,7 +998,14 @@ int io_import_fixed(int ddir, struct iov_iter *iter, struct io_rsrc_node *node,
*/
const struct bio_vec *bvec = imu->bvec;

- if (offset < bvec->bv_len) {
+ /*
+ * Kernel buffer bvecs, on the other hand, don't necessarily
+ * have the size property of user registered ones, so we have
+ * to use the slow iter advance.
+ */
+ if (node->type == IORING_RSRC_KBUF)
+ iov_iter_advance(iter, offset);
+ else if (offset < bvec->bv_len) {
iter->iov_offset = offset;
} else {
unsigned long seg_skip;
diff --git a/io_uring/rsrc.h b/io_uring/rsrc.h
index abd0d5d42c3e1..d1d90d9cd2b43 100644
--- a/io_uring/rsrc.h
+++ b/io_uring/rsrc.h
@@ -13,6 +13,7 @@
enum {
IORING_RSRC_FILE = 0,
IORING_RSRC_BUFFER = 1,
+ IORING_RSRC_KBUF = 2,
};

struct io_rsrc_node {
--
2.43.5