[PATCH 5.8 05/70] io_uring: stash ctx task reference for SQPOLL

Greg Kroah-Hartman
2020-10-31T11:40:52Z

From: Jens Axboe <axboe@kernel.dk>

commit 2aede0e417db846793c276c7a1bbf7262c8349b0 upstream.

We can grab a reference to the task instead of stashing away the task
files_struct. This is doable without creating a circular reference
between the ring fd and the task itself.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/io_uring.c | 39 +++++++++++++++++++++++++++++----------
1 file changed, 29 insertions(+), 10 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -264,7 +264,16 @@ struct io_ring_ctx {
/* IO offload */
struct io_wq *io_wq;
struct task_struct *sqo_thread; /* if using sq thread polling */
- struct mm_struct *sqo_mm;
+
+ /*
+ * For SQPOLL usage - we hold a reference to the parent task, so we
+ * have access to the ->files
+ */
+ struct task_struct *sqo_task;
+
+ /* Only used for accounting purposes */
+ struct mm_struct *mm_account;
+
wait_queue_head_t sqo_wait;

/*
@@ -4421,9 +4430,10 @@ static int io_sq_thread_acquire_mm(struc
{
if (io_op_defs[req->opcode].needs_mm && !current->mm) {
if (unlikely(!(ctx->flags & IORING_SETUP_SQPOLL) ||
- !mmget_not_zero(ctx->sqo_mm)))
+ !ctx->sqo_task->mm ||
+ !mmget_not_zero(ctx->sqo_task->mm)))
return -EFAULT;
- kthread_use_mm(ctx->sqo_mm);
+ kthread_use_mm(ctx->sqo_task->mm);
}

return 0;
@@ -7104,9 +7114,6 @@ static int io_sq_offload_start(struct io
{
int ret;

- mmgrab(current->mm);
- ctx->sqo_mm = current->mm;
-
if (ctx->flags & IORING_SETUP_SQPOLL) {
ret = -EPERM;
if (!capable(CAP_SYS_ADMIN))
@@ -7151,8 +7158,6 @@ static int io_sq_offload_start(struct io
return 0;
err:
io_finish_async(ctx);
- mmdrop(ctx->sqo_mm);
- ctx->sqo_mm = NULL;
return ret;
}

@@ -7482,8 +7487,12 @@ static void io_destroy_buffers(struct io
static void io_ring_ctx_free(struct io_ring_ctx *ctx)
{
io_finish_async(ctx);
- if (ctx->sqo_mm)
- mmdrop(ctx->sqo_mm);
+ if (ctx->sqo_task) {
+ put_task_struct(ctx->sqo_task);
+ ctx->sqo_task = NULL;
+ mmdrop(ctx->mm_account);
+ ctx->mm_account = NULL;
+ }

io_iopoll_reap_events(ctx);
io_sqe_buffer_unregister(ctx);
@@ -8256,6 +8265,16 @@ static int io_uring_create(unsigned entr
ctx->user = user;
ctx->creds = get_current_cred();

+ ctx->sqo_task = get_task_struct(current);
+ /*
+ * This is just grabbed for accounting purposes. When a process exits,
+ * the mm is exited and dropped before the files, hence we need to hang
+ * on to this mm purely for the purposes of being able to unaccount
+ * memory (locked/pinned vm). It's not used for anything else.
+ */
+ mmgrab(current->mm);
+ ctx->mm_account = current->mm;
+
ret = io_allocate_scq_urings(ctx, p);
if (ret)
goto err;