[PATCH 5.9 05/74] io_uring: stash ctx task reference for SQPOLL

Greg Kroah-Hartman
2020-10-31T11:44:09Z

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 | 47 ++++++++++++++++++++++++++++++++++-------------
1 file changed, 34 insertions(+), 13 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -265,7 +265,16 @@ 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;

/*
@@ -969,9 +978,10 @@ static int __io_sq_thread_acquire_mm(str
{
if (!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;
@@ -7591,11 +7601,11 @@ static void io_unaccount_mem(struct io_r
if (ctx->limit_mem)
__io_unaccount_mem(ctx->user, nr_pages);

- if (ctx->sqo_mm) {
+ if (ctx->mm_account) {
if (acct == ACCT_LOCKED)
- ctx->sqo_mm->locked_vm -= nr_pages;
+ ctx->mm_account->locked_vm -= nr_pages;
else if (acct == ACCT_PINNED)
- atomic64_sub(nr_pages, &ctx->sqo_mm->pinned_vm);
+ atomic64_sub(nr_pages, &ctx->mm_account->pinned_vm);
}
}

@@ -7610,11 +7620,11 @@ static int io_account_mem(struct io_ring
return ret;
}

- if (ctx->sqo_mm) {
+ if (ctx->mm_account) {
if (acct == ACCT_LOCKED)
- ctx->sqo_mm->locked_vm += nr_pages;
+ ctx->mm_account->locked_vm += nr_pages;
else if (acct == ACCT_PINNED)
- atomic64_add(nr_pages, &ctx->sqo_mm->pinned_vm);
+ atomic64_add(nr_pages, &ctx->mm_account->pinned_vm);
}

return 0;
@@ -7918,9 +7928,12 @@ static void io_ring_ctx_free(struct io_r
{
io_finish_async(ctx);
io_sqe_buffer_unregister(ctx);
- if (ctx->sqo_mm) {
- mmdrop(ctx->sqo_mm);
- ctx->sqo_mm = NULL;
+
+ if (ctx->sqo_task) {
+ put_task_struct(ctx->sqo_task);
+ ctx->sqo_task = NULL;
+ mmdrop(ctx->mm_account);
+ ctx->mm_account = NULL;
}

io_sqe_files_unregister(ctx);
@@ -8665,8 +8678,16 @@ 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->sqo_mm = current->mm;
+ ctx->mm_account = current->mm;

/*
* Account memory _before_ installing the file descriptor. Once