[RFC dma-buf 0/3] Improve the dma-buf tracking

[Chenbo Feng]

Currently, all dma-bufs share the same anonymous inode. While we can count
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 each
buffer.

To resolve the issue above and provide better method for userspace to track
the dma-buf usage across different processes, the following changes are
proposed in dma-buf kernel side. First of all, replace the singleton inode
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’s address 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, add
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 name
of each buffer.

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(-)

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2.21.0.rc2.261.ga7da99ff1b-goog