[PATCHv2 3/6] io_uring: add support for kernel registered bvecs

Keith Busch
2025-02-11T00:57:18Z

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 | 4 ++
io_uring/rsrc.c | 100 +++++++++++++++++++++++++++++++--
io_uring/rsrc.h | 1 +
4 files changed, 100 insertions(+), 6 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 e2fef264ff8b8..99aac2d52fbae 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -693,4 +693,8 @@ 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, struct request *rq,
+ void (*release)(void *), unsigned int index);
+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 30f08cf13ef60..14efec8587888 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -110,8 +110,9 @@ 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_BUFFER)
+ for (i = 0; i < imu->nr_bvecs; i++)
+ unpin_user_page(imu->bvec[i].bv_page);
if (imu->acct_pages)
io_unaccount_mem(ctx, imu->acct_pages);
kvfree(imu);
@@ -240,6 +241,13 @@ 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)) {
@@ -265,7 +273,6 @@ 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)
@@ -452,6 +459,7 @@ void io_free_rsrc_node(struct io_ring_ctx *ctx, struct io_rsrc_node *node)
if (io_slot_file(node))
fput(io_slot_file(node));
break;
+ case IORING_RSRC_KBUFFER:
case IORING_RSRC_BUFFER:
if (node->buf)
io_buffer_unmap(ctx, node);
@@ -862,6 +870,79 @@ int io_sqe_buffers_register(struct io_ring_ctx *ctx, void __user *arg,
return ret;
}

+int io_buffer_register_bvec(struct io_ring_ctx *ctx, struct request *rq,
+ void (*release)(void *), unsigned int index)
+{
+ struct io_rsrc_data *data = &ctx->buf_table;
+ struct req_iterator rq_iter;
+ struct io_mapped_ubuf *imu;
+ struct io_rsrc_node *node;
+ struct bio_vec bv;
+ u16 nr_bvecs;
+ int i = 0;
+
+ lockdep_assert_held(&ctx->uring_lock);
+
+ if (!data->nr)
+ return -EINVAL;
+ if (index >= data->nr)
+ return -EINVAL;
+
+ node = data->nodes[index];
+ if (node)
+ return -EBUSY;
+
+ node = io_rsrc_node_alloc(IORING_RSRC_KBUFFER);
+ if (!node)
+ return -ENOMEM;
+
+ node->release = release;
+ node->priv = rq;
+
+ nr_bvecs = blk_rq_nr_phys_segments(rq);
+ imu = kvmalloc(struct_size(imu, bvec, nr_bvecs), GFP_KERNEL);
+ if (!imu) {
+ kfree(node);
+ return -ENOMEM;
+ }
+
+ imu->ubuf = 0;
+ imu->len = blk_rq_bytes(rq);
+ imu->acct_pages = 0;
+ imu->nr_bvecs = nr_bvecs;
+ refcount_set(&imu->refs, 1);
+ node->buf = imu;
+
+ rq_for_each_bvec(bv, rq, rq_iter)
+ bvec_set_page(&node->buf->bvec[i++], bv.bv_page, bv.bv_len,
+ bv.bv_offset);
+ 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 (!data->nr)
+ return;
+ if (index >= data->nr)
+ return;
+
+ node = data->nodes[index];
+ if (!node || !node->buf)
+ return;
+ if (node->type != IORING_RSRC_KBUFFER)
+ 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)
{
@@ -888,8 +969,8 @@ 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
@@ -903,8 +984,15 @@ int io_import_fixed(int ddir, struct iov_iter *iter, struct io_rsrc_node *node,
*/
const struct bio_vec *bvec = imu->bvec;

+ /*
+ * 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 (offset < bvec->bv_len) {
iter->iov_offset = offset;
+ } else if (node->type == IORING_RSRC_KBUFFER) {
+ iov_iter_advance(iter, offset);
} else {
unsigned long seg_skip;

@@ -1004,7 +1092,7 @@ static int io_clone_buffers(struct io_ring_ctx *ctx, struct io_ring_ctx *src_ctx
if (!src_node) {
dst_node = NULL;
} else {
- dst_node = io_rsrc_node_alloc(IORING_RSRC_BUFFER);
+ dst_node = io_rsrc_node_alloc(src_node->type);
if (!dst_node) {
ret = -ENOMEM;
goto out_free;
diff --git a/io_uring/rsrc.h b/io_uring/rsrc.h
index a3826ab84e666..8147dfc26f737 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_KBUFFER = 2,
};

struct io_rsrc_node {
--
2.43.5