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    Subject[PATCH 5.7 433/477] io_uring: acquire mm for task_work for SQPOLL
    Date
    From: Jens Axboe <axboe@kernel.dk>

    [ Upstream commit 9d8426a09195e2dcf2aa249de2aaadd792d491c7 ]

    If we're unlucky with timing, we could be running task_work after
    having dropped the memory context in the sq thread. Since dropping
    the context requires a runnable task state, we cannot reliably drop
    it as part of our check-for-work loop in io_sq_thread(). Instead,
    abstract out the mm acquire for the sq thread into a helper, and call
    it from the async task work handler.

    Cc: stable@vger.kernel.org # v5.7
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/io_uring.c | 44 +++++++++++++++++++++++++++++---------------
    1 file changed, 29 insertions(+), 15 deletions(-)

    --- a/fs/io_uring.c
    +++ b/fs/io_uring.c
    @@ -4302,6 +4302,28 @@ static void io_async_queue_proc(struct f
    __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) {
    + 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;
    + 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);
    @@ -4333,12 +4355,17 @@ static void io_async_task_func(struct ca
    if (canceled) {
    kfree(apoll);
    io_cqring_ev_posted(ctx);
    +end_req:
    req_set_fail_links(req);
    io_double_put_req(req);
    return;
    }

    __set_current_state(TASK_RUNNING);
    + if (io_sq_thread_acquire_mm(ctx, req)) {
    + io_cqring_add_event(req, -EFAULT);
    + goto end_req;
    + }
    mutex_lock(&ctx->uring_lock);
    __io_queue_sqe(req, NULL);
    mutex_unlock(&ctx->uring_lock);
    @@ -5897,11 +5924,8 @@ static int io_init_req(struct io_ring_ct
    if (unlikely(req->opcode >= IORING_OP_LAST))
    return -EINVAL;

    - if (io_op_defs[req->opcode].needs_mm && !current->mm) {
    - if (unlikely(!mmget_not_zero(ctx->sqo_mm)))
    - return -EFAULT;
    - use_mm(ctx->sqo_mm);
    - }
    + if (unlikely(io_sq_thread_acquire_mm(ctx, req)))
    + return -EFAULT;

    sqe_flags = READ_ONCE(sqe->flags);
    /* enforce forwards compatibility on users */
    @@ -6011,16 +6035,6 @@ fail_req:
    return submitted;
    }

    -static inline void io_sq_thread_drop_mm(struct io_ring_ctx *ctx)
    -{
    - struct mm_struct *mm = current->mm;
    -
    - if (mm) {
    - unuse_mm(mm);
    - mmput(mm);
    - }
    -}
    -
    static int io_sq_thread(void *data)
    {
    struct io_ring_ctx *ctx = data;

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