dmabuf: Add the capability to expose DMA-BUF stats in sysfs

Overview
========
The patch adds DMA-BUF statistics to /sys/kernel/dmabuf/buffers. It
allows statistics to be enabled for each DMA-BUF in sysfs by enabling
the config CONFIG_DMABUF_SYSFS_STATS.

The following stats will be exposed by the interface:

/sys/kernel/dmabuf/buffers/<inode_number>/exporter_name
/sys/kernel/dmabuf/buffers/<inode_number>/size
/sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attach_uid>/device
/sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attach_uid>/map_counter

The inode_number is unique for each DMA-BUF and was added earlier [1]
in order to allow userspace to track DMA-BUF usage across different
processes.

Use Cases
=========
The interface provides a way to gather DMA-BUF per-buffer statistics
from production devices. These statistics will be used to derive DMA-BUF
per-exporter stats and per-device usage stats for Android Bug reports.
The corresponding userspace changes can be found at [2].
Telemetry tools will also capture this information(along with other
memory metrics) periodically as well as on important events like a
foreground app kill (which might have been triggered by Low Memory
Killer). It will also contribute to provide a snapshot of the system
memory usage on other events such as OOM kills and Application Not
Responding events.

Background
==========
Currently, there are two existing interfaces that provide information
about DMA-BUFs.
1) /sys/kernel/debug/dma_buf/bufinfo
debugfs is however unsuitable to be mounted in production systems and
cannot be considered as an alternative to the sysfs interface being
proposed.
2) proc/<pid>/fdinfo/<fd>
The proc/<pid>/fdinfo/<fd> files expose information about DMA-BUF fds.
However, the existing procfs interfaces can only provide information
about the buffers for which processes hold fds or have the buffers
mmapped into their address space. Since the procfs interfaces alone
cannot provide a full picture of all DMA-BUFs in the system, there is
the need for an alternate interface to provide this information on
production systems.

The patch contains the following major improvements over v1:
1) Each attachment is represented by its own directory to allow creating
a symlink to the importing device and to also provide room for future
expansion.
2) The number of distinct mappings of each attachment is exposed in a
separate file.
3) The per-buffer statistics are now in /sys/kernel/dmabuf/buffers
inorder to make the interface expandable in future.

All of the improvements above are based on suggestions/feedback from
Daniel Vetter and Christian König.

A shell script that can be run on a classic Linux environment to read
out the DMA-BUF statistics can be found at [3](suggested by John
Stultz).

[1]: https://lore.kernel.org/patchwork/patch/1088791/
[2]: https://android-review.googlesource.com/q/topic:%22dmabuf-sysfs%22+(status:open%20OR%20status:merged)
[3]: https://android-review.googlesource.com/c/platform/system/memory/libmeminfo/+/1549734

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603214758.2955251-1-hridya@google.com

2 files changed, 57 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers b/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
new file mode 100644
index 000000000000..a243984ed420
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-kernel-dmabuf-buffers
@@ -0,0 +1,52 @@
+What:		/sys/kernel/dmabuf/buffers
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	The /sys/kernel/dmabuf/buffers directory contains a
+		snapshot of the internal state of every DMA-BUF.
+		/sys/kernel/dmabuf/buffers/<inode_number> will contain the
+		statistics for the DMA-BUF with the unique inode number
+		<inode_number>
+Users:		kernel memory tuning/debugging tools
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/exporter_name
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This file is read-only and contains the name of the exporter of
+		the DMA-BUF.
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/size
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This file is read-only and specifies the size of the DMA-BUF in
+		bytes.
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/attachments
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This directory will contain subdirectories representing every
+		attachment of the DMA-BUF.
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attachment_uid>
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This directory will contain information on the attached device
+		and the number of current distinct device mappings.
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attachment_uid>/device
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This file is read-only and is a symlink to the attached device's
+		sysfs entry.
+
+What:		/sys/kernel/dmabuf/buffers/<inode_number>/attachments/<attachment_uid>/map_counter
+Date:		May 2021
+KernelVersion:	v5.13
+Contact:	Hridya Valsaraju <hridya@google.com>
+Description:	This file is read-only and contains a map_counter indicating the
+		number of distinct device mappings of the attachment.
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 7f21425d9435..84aab65c4962 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -106,6 +106,11 @@ Implicit Fence Poll Support
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :doc: implicit fence polling
 
+DMA-BUF statistics
+~~~~~~~~~~~~~~~~~~
+.. kernel-doc:: drivers/dma-buf/dma-buf-sysfs-stats.c
+   :doc: overview
+
 Kernel Functions and Structures Reference
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~