[PATCHv8 4/6] io_uring: add support for kernel registered bvecs

Keith Busch
2025-02-27T22:39:57Z

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/cmd.h | 7 ++
io_uring/io_uring.c | 3 +
io_uring/rsrc.c | 123 +++++++++++++++++++++++++++++++++--
io_uring/rsrc.h | 9 +++
io_uring/rw.c | 3 +
5 files changed, 138 insertions(+), 7 deletions(-)

diff --git a/include/linux/io_uring/cmd.h b/include/linux/io_uring/cmd.h
index 87150dc0a07cf..cf8d80d847344 100644
--- a/include/linux/io_uring/cmd.h
+++ b/include/linux/io_uring/cmd.h
@@ -4,6 +4,7 @@

#include <uapi/linux/io_uring.h>
#include <linux/io_uring_types.h>
+#include <linux/blk-mq.h>

/* only top 8 bits of sqe->uring_cmd_flags for kernel internal use */
#define IORING_URING_CMD_CANCELABLE (1U << 30)
@@ -125,4 +126,10 @@ static inline struct io_uring_cmd_data *io_uring_cmd_get_async_data(struct io_ur
return cmd_to_io_kiocb(cmd)->async_data;
}

+int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
+ void (*release)(void *), unsigned int index,
+ unsigned int issue_flags);
+void io_buffer_unregister_bvec(struct io_uring_cmd *cmd, unsigned int index,
+ unsigned int issue_flags);
+
#endif /* _LINUX_IO_URING_CMD_H */
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index db1c0792def63..2f5dd47e7dbf5 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3947,6 +3947,9 @@ static int __init io_uring_init(void)

io_uring_optable_init();

+ /* imu->dir is u8 */
+ BUILD_BUG_ON((IO_IMU_DEST | IO_IMU_SOURCE) > U8_MAX);
+
/*
* Allow user copy in the per-command field, which starts after the
* file in io_kiocb and until the opcode field. The openat2 handling
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index f814526982c36..0eceaf2e03777 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -9,6 +9,7 @@
#include <linux/hugetlb.h>
#include <linux/compat.h>
#include <linux/io_uring.h>
+#include <linux/io_uring/cmd.h>

#include <uapi/linux/io_uring.h>

@@ -101,17 +102,23 @@ int io_buffer_validate(struct iovec *iov)
return 0;
}

-static void io_buffer_unmap(struct io_ring_ctx *ctx, struct io_rsrc_node *node)
+static void io_release_ubuf(void *priv)
{
- struct io_mapped_ubuf *imu = node->buf;
+ struct io_mapped_ubuf *imu = priv;
unsigned int i;

- if (!refcount_dec_and_test(&imu->refs))
- return;
for (i = 0; i < imu->nr_bvecs; i++)
unpin_user_page(imu->bvec[i].bv_page);
+}
+
+static void io_buffer_unmap(struct io_ring_ctx *ctx, struct io_mapped_ubuf *imu)
+{
+ if (!refcount_dec_and_test(&imu->refs))
+ return;
+
if (imu->acct_pages)
io_unaccount_mem(ctx, imu->acct_pages);
+ imu->release(imu->priv);
kvfree(imu);
}

@@ -451,7 +458,7 @@ void io_free_rsrc_node(struct io_ring_ctx *ctx, struct io_rsrc_node *node)
break;
case IORING_RSRC_BUFFER:
if (node->buf)
- io_buffer_unmap(ctx, node);
+ io_buffer_unmap(ctx, node->buf);
break;
default:
WARN_ON_ONCE(1);
@@ -761,6 +768,10 @@ static struct io_rsrc_node *io_sqe_buffer_register(struct io_ring_ctx *ctx,
imu->len = iov->iov_len;
imu->nr_bvecs = nr_pages;
imu->folio_shift = PAGE_SHIFT;
+ imu->release = io_release_ubuf;
+ imu->priv = imu;
+ imu->is_kbuf = false;
+ imu->dir = IO_IMU_DEST | IO_IMU_SOURCE;
if (coalesced)
imu->folio_shift = data.folio_shift;
refcount_set(&imu->refs, 1);
@@ -857,6 +868,95 @@ int io_sqe_buffers_register(struct io_ring_ctx *ctx, void __user *arg,
return ret;
}

+int io_buffer_register_bvec(struct io_uring_cmd *cmd, struct request *rq,
+ void (*release)(void *), unsigned int index,
+ unsigned int issue_flags)
+{
+ struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
+ 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, *bvec;
+ u16 nr_bvecs;
+ int ret = 0;
+
+ io_ring_submit_lock(ctx, issue_flags);
+ if (index >= data->nr) {
+ ret = -EINVAL;
+ goto unlock;
+ }
+ index = array_index_nospec(index, data->nr);
+
+ if (data->nodes[index]) {
+ ret = -EBUSY;
+ goto unlock;
+ }
+
+ node = io_rsrc_node_alloc(IORING_RSRC_BUFFER);
+ if (!node) {
+ ret = -ENOMEM;
+ goto unlock;
+ }
+
+ nr_bvecs = blk_rq_nr_phys_segments(rq);
+ imu = kvmalloc(struct_size(imu, bvec, nr_bvecs), GFP_KERNEL);
+ if (!imu) {
+ kfree(node);
+ ret = -ENOMEM;
+ goto unlock;
+ }
+
+ imu->ubuf = 0;
+ imu->len = blk_rq_bytes(rq);
+ imu->acct_pages = 0;
+ imu->folio_shift = PAGE_SHIFT;
+ imu->nr_bvecs = nr_bvecs;
+ refcount_set(&imu->refs, 1);
+ imu->release = release;
+ imu->priv = rq;
+ imu->is_kbuf = true;
+
+ if (op_is_write(req_op(rq)))
+ imu->dir = IO_IMU_SOURCE;
+ else
+ imu->dir = IO_IMU_DEST;
+
+ bvec = imu->bvec;
+ rq_for_each_bvec(bv, rq, rq_iter)
+ *bvec++ = bv;
+
+ node->buf = imu;
+ data->nodes[index] = node;
+unlock:
+ io_ring_submit_unlock(ctx, issue_flags);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(io_buffer_register_bvec);
+
+void io_buffer_unregister_bvec(struct io_uring_cmd *cmd, unsigned int index,
+ unsigned int issue_flags)
+{
+ struct io_ring_ctx *ctx = cmd_to_io_kiocb(cmd)->ctx;
+ struct io_rsrc_data *data = &ctx->buf_table;
+ struct io_rsrc_node *node;
+
+ io_ring_submit_lock(ctx, issue_flags);
+ if (index >= data->nr)
+ goto unlock;
+ index = array_index_nospec(index, data->nr);
+
+ node = data->nodes[index];
+ if (!node || !node->buf->is_kbuf)
+ goto unlock;
+
+ io_put_rsrc_node(ctx, node);
+ data->nodes[index] = NULL;
+unlock:
+ io_ring_submit_unlock(ctx, issue_flags);
+}
+EXPORT_SYMBOL_GPL(io_buffer_unregister_bvec);
+
static int io_import_fixed(int ddir, struct iov_iter *iter,
struct io_mapped_ubuf *imu,
u64 buf_addr, size_t len)
@@ -871,6 +971,8 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
/* not inside the mapped region */
if (unlikely(buf_addr < imu->ubuf || buf_end > (imu->ubuf + imu->len)))
return -EFAULT;
+ if (!(imu->dir & (1 << ddir)))
+ return -EFAULT;

/*
* Might not be a start of buffer, set size appropriately
@@ -883,8 +985,8 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
/*
* 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
@@ -898,8 +1000,15 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
*/
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 (imu->is_kbuf) {
+ iov_iter_advance(iter, offset);
} else {
unsigned long seg_skip;

diff --git a/io_uring/rsrc.h b/io_uring/rsrc.h
index f0e9080599646..7600e2736eeb3 100644
--- a/io_uring/rsrc.h
+++ b/io_uring/rsrc.h
@@ -20,6 +20,11 @@ struct io_rsrc_node {
};
};

+enum {
+ IO_IMU_DEST = 1 << ITER_DEST,
+ IO_IMU_SOURCE = 1 << ITER_SOURCE,
+};
+
struct io_mapped_ubuf {
u64 ubuf;
unsigned int len;
@@ -27,6 +32,10 @@ struct io_mapped_ubuf {
unsigned int folio_shift;
refcount_t refs;
unsigned long acct_pages;
+ void (*release)(void *);
+ void *priv;
+ bool is_kbuf;
+ u8 dir;
struct bio_vec bvec[] __counted_by(nr_bvecs);
};

diff --git a/io_uring/rw.c b/io_uring/rw.c
index 7bc23802a388e..5ee9f8949e8ba 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -629,6 +629,7 @@ static inline loff_t *io_kiocb_ppos(struct kiocb *kiocb)
*/
static ssize_t loop_rw_iter(int ddir, struct io_rw *rw, struct iov_iter *iter)
{
+ struct io_kiocb *req = cmd_to_io_kiocb(rw);
struct kiocb *kiocb = &rw->kiocb;
struct file *file = kiocb->ki_filp;
ssize_t ret = 0;
@@ -644,6 +645,8 @@ static ssize_t loop_rw_iter(int ddir, struct io_rw *rw, struct iov_iter *iter)
if ((kiocb->ki_flags & IOCB_NOWAIT) &&
!(kiocb->ki_filp->f_flags & O_NONBLOCK))
return -EAGAIN;
+ if ((req->flags & REQ_F_BUF_NODE) && req->buf_node->buf->is_kbuf)
+ return -EFAULT;

ppos = io_kiocb_ppos(kiocb);

--
2.43.5