Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190227035448.117169-1-fengc@google.com>
In-Reply-To: <20190227035448.117169-1-fengc@google.com>
From:   Sumit Semwal <sumit.semwal@linaro.org>
Date:   Thu, 14 Mar 2019 23:19:06 +0530
Message-ID: <CAO_48GG0sW3AJNVJnydjuWUQHcmWe5aQrB=X0Wo7uBtDnPtoYg@mail.gmail.com>
Subject: Re: [RFC dma-buf 0/3] Improve the dma-buf tracking
To:     Chenbo Feng <fengc@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        DRI mailing list <dri-devel@lists.freedesktop.org>,
        linux-media@vger.kernel.org, erickreyes@google.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hello Chenbo,Thank you for your RFC series.

On Wed, 27 Feb 2019 at 09:24, Chenbo Feng <fengc@google.com> wrote:
>
> Currently, all dma-bufs share the same anonymous inode. While we can coun=
t
> how many dma-buf fds or mappings a process has, we can't get the size of
> the backing buffers or tell if two entries point to the same dma-buf. And
> in debugfs, we can get a per-buffer breakdown of size and reference count=
,
> but can't tell which processes are actually holding the references to eac=
h
> buffer.
>
> To resolve the issue above and provide better method for userspace to tra=
ck
> the dma-buf usage across different processes, the following changes are
> proposed in dma-buf kernel side. First of all, replace the singleton inod=
e
> inside the dma-buf subsystem with a mini-filesystem, and assign each
> dma-buf a unique inode out of this filesystem.  With this change, calling
> stat(2) on each entry gives the caller a unique ID (st_ino), the buffer's
> size (st_size), and even the number of pages assigned to each dma-buffer.
> Secoundly, add the inode information to /sys/kernel/debug/dma_buf/bufinfo
> so in the case where a buffer is mmap()ed into a process=E2=80=99s addres=
s space
> but all remaining fds have been closed, we can still get the dma-buf
> information and try to accociate it with the process by searching the
> proc/pid/maps and looking for the corresponding inode number exposed in
> dma-buf debug fs. Thirdly, created an ioctl to assign names to dma-bufs
> which lets userspace assign short names (e.g., "CAMERA") to buffers. This
> information can be extremely helpful for tracking and accounting shared
> buffers based on their usage and original purpose. Last but not least, ad=
d
> dma-buf information to /proc/pid/fdinfo by adding a show_fdinfo() handler
> to dma_buf_file_operations. The handler will print the file_count and nam=
e
> of each buffer.

In general, I think I like the idea as it contributes to a much more
relevant usage analysis of dma-buf backed buffers.
I will get to doing a more detailed review soon, but immediately, we
might want to think a bit about the get/set_name IOCTLS - do we need
to think of disallowing multiple renaming of buffers once they start
getting used? It could otherwise make the whole metrics a lot
confused?

>
> Greg Hackmann (3):
>   dma-buf: give each buffer a full-fledged inode
>   dma-buf: add DMA_BUF_{GET,SET}_NAME ioctls
>   dma-buf: add show_fdinfo handler
>
>  drivers/dma-buf/dma-buf.c    | 121 ++++++++++++++++++++++++++++++++---
>  include/linux/dma-buf.h      |   5 +-
>  include/uapi/linux/dma-buf.h |   4 ++
>  include/uapi/linux/magic.h   |   1 +
>  4 files changed, 122 insertions(+), 9 deletions(-)
>
> --
> 2.21.0.rc2.261.ga7da99ff1b-goog
>

Best,
Sumit.