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

[Greg Kroah-Hartman]

From: Jens Axboe <ax...@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.sile...@gmail.com>
Signed-off-by: Jens Axboe <ax...@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gre...@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;