Re: [RFC v2 6/6] android: binder: Add a buffer flag to relinquish ownership of fds

["T.J. Mercier" <tjmercier@google.com>]

On Mon, Feb 14, 2022 at 1:26 PM Todd Kjos <tkjos@google.com> wrote:
>
> On Fri, Feb 11, 2022 at 8:19 AM T.J. Mercier <tjmercier@google.com> wrote:
> >
> > This patch introduces a buffer flag BINDER_BUFFER_FLAG_SENDER_NO_NEED
> > that a process sending an fd array to another process over binder IPC
> > can set to relinquish ownership of the fds being sent for memory
> > accounting purposes. If the flag is found to be set during the fd array
> > translation and the fd is for a DMA-BUF, the buffer is uncharged from
> > the sender's cgroup and charged to the receiving process's cgroup
> > instead.
> >
> > It is up to the sending process to ensure that it closes the fds
> > regardless of whether the transfer failed or succeeded.
> >
> > Most graphics shared memory allocations in Android are done by the
> > graphics allocator HAL process. On requests from clients, the HAL process
> > allocates memory and sends the fds to the clients over binder IPC.
> > The graphics allocator HAL will not retain any references to the
> > buffers. When the HAL sets the BINDER_BUFFER_FLAG_SENDER_NO_NEED for fd
> > arrays holding DMA-BUF fds, the gpu cgroup controller will be able to
> > correctly charge the buffers to the client processes instead of the
> > graphics allocator HAL.
> >
> > From: Hridya Valsaraju <hridya@google.com>
> > Signed-off-by: Hridya Valsaraju <hridya@google.com>
> > Co-developed-by: T.J. Mercier <tjmercier@google.com>
> > Signed-off-by: T.J. Mercier <tjmercier@google.com>
> > ---
> > changes in v2
> > - Move dma-buf cgroup charge transfer from a dma_buf_op defined by every
> > heap to a single dma-buf function for all heaps per Daniel Vetter and
> > Christian König.
> >
> >  drivers/android/binder.c            | 26 ++++++++++++++++++++++++++
> >  include/uapi/linux/android/binder.h |  1 +
> >  2 files changed, 27 insertions(+)
> >
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > index 8351c5638880..f50d88ded188 100644
> > --- a/drivers/android/binder.c
> > +++ b/drivers/android/binder.c
> > @@ -42,6 +42,7 @@
> >
> >  #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> >
> > +#include <linux/dma-buf.h>
> >  #include <linux/fdtable.h>
> >  #include <linux/file.h>
> >  #include <linux/freezer.h>
> > @@ -2482,8 +2483,10 @@ static int binder_translate_fd_array(struct list_head *pf_head,
>
> Is this only needed for the BINDER_TYPE_FDA case (multiple fds)? This
> never needs to be done in the BINDER_TYPE_FD case (single fd)?
>

Currently this is the case as there is no user who would benefit from
the single fd case. The only known user is the gralloc HAL which
always uses BINDER_TYPE_FDA to send dmabufs. I guess we could move the
code into binder_translate_fd if we were willing to bring back
binder_fd_object's flags field. This looks possible, but I think it'd
be a more intrusive change.

> >  {
> >         binder_size_t fdi, fd_buf_size;
> >         binder_size_t fda_offset;
> > +       bool transfer_gpu_charge = false;
> >         const void __user *sender_ufda_base;
> >         struct binder_proc *proc = thread->proc;
> > +       struct binder_proc *target_proc = t->to_proc;
> >         int ret;
> >
> >         fd_buf_size = sizeof(u32) * fda->num_fds;
> > @@ -2521,8 +2524,15 @@ static int binder_translate_fd_array(struct list_head *pf_head,
> >         if (ret)
> >                 return ret;
> >
> > +       if (IS_ENABLED(CONFIG_CGROUP_GPU) &&
> > +               parent->flags & BINDER_BUFFER_FLAG_SENDER_NO_NEED)
> > +               transfer_gpu_charge = true;
> > +
> >         for (fdi = 0; fdi < fda->num_fds; fdi++) {
> >                 u32 fd;
> > +               struct dma_buf *dmabuf;
> > +               struct gpucg *gpucg;
> > +
> >                 binder_size_t offset = fda_offset + fdi * sizeof(fd);
> >                 binder_size_t sender_uoffset = fdi * sizeof(fd);
> >
> > @@ -2532,6 +2542,22 @@ static int binder_translate_fd_array(struct list_head *pf_head,
> >                                                   in_reply_to);
> >                 if (ret)
> >                         return ret > 0 ? -EINVAL : ret;
> > +
> > +               if (!transfer_gpu_charge)
> > +                       continue;
> > +
> > +               dmabuf = dma_buf_get(fd);
> > +               if (IS_ERR(dmabuf))
> > +                       continue;
> > +
> > +               gpucg = gpucg_get(target_proc->tsk);
> > +               ret = dma_buf_charge_transfer(dmabuf, gpucg);
> > +               if (ret) {
> > +                       pr_warn("%d:%d Unable to transfer DMA-BUF fd charge to %d",
> > +                               proc->pid, thread->pid, target_proc->pid);
> > +                       gpucg_put(gpucg);
> > +               }
> > +               dma_buf_put(dmabuf);
> >         }
> >         return 0;
> >  }
> > diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
> > index 3246f2c74696..169fd5069a1a 100644
> > --- a/include/uapi/linux/android/binder.h
> > +++ b/include/uapi/linux/android/binder.h
> > @@ -137,6 +137,7 @@ struct binder_buffer_object {
> >
> >  enum {
> >         BINDER_BUFFER_FLAG_HAS_PARENT = 0x01,
> > +       BINDER_BUFFER_FLAG_SENDER_NO_NEED = 0x02,
> >  };
> >
> >  /* struct binder_fd_array_object - object describing an array of fds in a buffer
> > --
> > 2.35.1.265.g69c8d7142f-goog
> >