[PATCH 04/15] io_uring: re-issue block requests that failed because of resources

Jens Axboe
2020-06-18T14:46:19Z

Mark the plug with nowait == true, which will cause requests to avoid
blocking on request allocation. If they do, we catch them and reissue
them from a task_work based handler.

Normally we can catch -EAGAIN directly, but the hard case is for split
requests. As an example, the application issues a 512KB request. The
block core will split this into 128KB if that's the max size for the
device. The first request issues just fine, but we run into -EAGAIN for
some latter splits for the same request. As the bio is split, we don't
get to see the -EAGAIN until one of the actual reads complete, and hence
we cannot handle it inline as part of submission.

This does potentially cause re-reads of parts of the range, as the whole
request is reissued. There's currently no better way to handle this.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
fs/io_uring.c | 148 ++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 124 insertions(+), 24 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2e257c5a1866..40413fb9d07b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -900,6 +900,13 @@ static int io_file_get(struct io_submit_state *state, struct io_kiocb *req,
static void __io_queue_sqe(struct io_kiocb *req,
const struct io_uring_sqe *sqe);

+static ssize_t io_import_iovec(int rw, struct io_kiocb *req,
+ struct iovec **iovec, struct iov_iter *iter,
+ bool needs_lock);
+static int io_setup_async_rw(struct io_kiocb *req, ssize_t io_size,
+ struct iovec *iovec, struct iovec *fast_iov,
+ struct iov_iter *iter);
+
static struct kmem_cache *req_cachep;

static const struct file_operations io_uring_fops;
@@ -1978,12 +1985,115 @@ static void io_complete_rw_common(struct kiocb *kiocb, long res)
__io_cqring_add_event(req, res, cflags);
}

+static void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
+{
+ struct mm_struct *mm = current->mm;
+
+ if (mm) {
+ kthread_unuse_mm(mm);
+ mmput(mm);
+ }
+}
+
+static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
+ struct io_kiocb *req)
+{
+ if (io_op_defs[req->opcode].needs_mm && !current->mm) {
+ if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
+ return -EFAULT;
+ kthread_use_mm(ctx->sqo_mm);
+ }
+
+ return 0;
+}
+
+#ifdef CONFIG_BLOCK
+static bool io_resubmit_prep(struct io_kiocb *req, int error)
+{
+ struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
+ ssize_t ret = -ECANCELED;
+ struct iov_iter iter;
+ int rw;
+
+ if (error) {
+ ret = error;
+ goto end_req;
+ }
+
+ switch (req->opcode) {
+ case IORING_OP_READV:
+ case IORING_OP_READ_FIXED:
+ case IORING_OP_READ:
+ rw = READ;
+ break;
+ case IORING_OP_WRITEV:
+ case IORING_OP_WRITE_FIXED:
+ case IORING_OP_WRITE:
+ rw = WRITE;
+ break;
+ default:
+ printk_once(KERN_WARNING "io_uring: bad opcode in resubmit %d\n",
+ req->opcode);
+ goto end_req;
+ }
+
+ ret = io_import_iovec(rw, req, &iovec, &iter, false);
+ if (ret < 0)
+ goto end_req;
+ ret = io_setup_async_rw(req, ret, iovec, inline_vecs, &iter);
+ if (!ret)
+ return true;
+ kfree(iovec);
+end_req:
+ io_cqring_add_event(req, ret);
+ req_set_fail_links(req);
+ io_put_req(req);
+ return false;
+}
+
+static void io_rw_resubmit(struct callback_head *cb)
+{
+ struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
+ struct io_ring_ctx *ctx = req->ctx;
+ int err;
+
+ __set_current_state(TASK_RUNNING);
+
+ err = io_sq_thread_acquire_mm(ctx, req);
+
+ if (io_resubmit_prep(req, err)) {
+ refcount_inc(&req->refs);
+ io_queue_async_work(req);
+ }
+}
+#endif
+
+static bool io_rw_reissue(struct io_kiocb *req, long res)
+{
+#ifdef CONFIG_BLOCK
+ struct task_struct *tsk;
+ int ret;
+
+ if ((res != -EAGAIN && res != -EOPNOTSUPP) || io_wq_current_is_worker())
+ return false;
+
+ tsk = req->task;
+ init_task_work(&req->task_work, io_rw_resubmit);
+ ret = task_work_add(tsk, &req->task_work, true);
+ if (!ret)
+ return true;
+#endif
+ return false;
+}
+
static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
{
struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw.kiocb);

- io_complete_rw_common(kiocb, res);
- io_put_req(req);
+ if (!io_rw_reissue(req, res)) {
+ io_complete_rw_common(kiocb, res);
+ io_put_req(req);
+ }
}

static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
@@ -2169,6 +2279,9 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
if (kiocb->ki_flags & IOCB_NOWAIT)
req->flags |= REQ_F_NOWAIT;

+ if (kiocb->ki_flags & IOCB_DIRECT)
+ io_get_req_task(req);
+
if (force_nonblock)
kiocb->ki_flags |= IOCB_NOWAIT;

@@ -2668,6 +2781,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
iov_count = iov_iter_count(&iter);
ret = rw_verify_area(READ, req->file, &kiocb->ki_pos, iov_count);
if (!ret) {
+ unsigned long nr_segs = iter.nr_segs;
ssize_t ret2 = 0;

if (req->file->f_op->read_iter)
@@ -2679,6 +2793,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
if (!force_nonblock || (ret2 != -EAGAIN && ret2 != -EIO)) {
kiocb_done(kiocb, ret2);
} else {
+ iter.count = iov_count;
+ iter.nr_segs = nr_segs;
copy_iov:
ret = io_setup_async_rw(req, io_size, iovec,
inline_vecs, &iter);
@@ -2765,6 +2881,7 @@ static int io_write(struct io_kiocb *req, bool force_nonblock)
iov_count = iov_iter_count(&iter);
ret = rw_verify_area(WRITE, req->file, &kiocb->ki_pos, iov_count);
if (!ret) {
+ unsigned long nr_segs = iter.nr_segs;
ssize_t ret2;

/*
@@ -2802,6 +2919,8 @@ static int io_write(struct io_kiocb *req, bool force_nonblock)
if (!force_nonblock || ret2 != -EAGAIN) {
kiocb_done(kiocb, ret2);
} else {
+ iter.count = iov_count;
+ iter.nr_segs = nr_segs;
copy_iov:
ret = io_setup_async_rw(req, io_size, iovec,
inline_vecs, &iter);
@@ -4282,28 +4401,6 @@ static void io_async_queue_proc(struct file *file, struct wait_queue_head *head,
__io_queue_proc(&pt->req->apoll->poll, pt, head);
}

-static void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
-{
- struct mm_struct *mm = current->mm;
-
- if (mm) {
- kthread_unuse_mm(mm);
- mmput(mm);
- }
-}
-
-static int io_sq_thread_acquire_mm(struct io_ring_ctx *ctx,
- struct io_kiocb *req)
-{
- if (io_op_defs[req->opcode].needs_mm && !current->mm) {
- if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
- return -EFAULT;
- kthread_use_mm(ctx->sqo_mm);
- }
-
- return 0;
-}
-
static void io_async_task_func(struct callback_head *cb)
{
struct io_kiocb *req = container_of(cb, struct io_kiocb, task_work);
@@ -5814,6 +5911,9 @@ static void io_submit_state_start(struct io_submit_state *state,
unsigned int max_ios)
{
blk_start_plug(&state->plug);
+#ifdef CONFIG_BLOCK
+ state->plug.nowait = true;
+#endif
state->free_reqs = 0;
state->file = NULL;
state->ios_left = max_ios;
--
2.27.0