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io_uring: re-issue block requests that failed because of resources
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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>
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@axboe
axboe committed Jun 22, 2020
1 parent 4503b76 commit b63534c
Showing 1 changed file with 124 additions and 24 deletions.
148 changes: 124 additions & 24 deletions fs/io_uring.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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;

Expand Down Expand Up @@ -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)
Expand All @@ -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);
Expand Down Expand Up @@ -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;

/*
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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;
Expand Down

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