[PATCH v22 0/5] Introducing trace buffer mapping by user-space

Vincent Donnefort
2024-04-30T11:13:59Z

The tracing ring-buffers can be stored on disk or sent to network
without any copy via splice. However the later doesn't allow real time
processing of the traces. A solution is to give userspace direct access
to the ring-buffer pages via a mapping. An application can now become a
consumer of the ring-buffer, in a similar fashion to what trace_pipe
offers.

Support for this new feature can already be found in libtracefs from
version 1.8, when built with EXTRA_CFLAGS=-DFORCE_MMAP_ENABLE.

Vincent

v21 -> v22:
* Remove DONTDUMP (VM_IO implies DONTDUMP already)
* Remove MIXEDMAP (implicit when using vm_insert_page)
* Remove PFNMAP (We do not perform raw PFN mappings and MIXEDMAP is
already implicitely set)
* Add comments to justify the VM_* flags

v20 -> v21:
* Collect Ack
* Add .gitignore
* Few nits
* Remove meta-page padding (zero-page not supported by vm_insert_pages)
* Remove single-usage macros
* Move vma flags handling into ring-buffer.c

v19 -> v20:
* Fix typos in documentation.
* Remove useless mmap open and fault callbacks.
* add mm.h include for vm_insert_pages

v18 -> v19:
* Use VM_PFNMAP and vm_insert_pages
* Allocate ring-buffer subbufs with __GFP_COMP
* Pad the meta-page with the zero-page to align on the subbuf_order
* Extend the ring-buffer test with mmap() dedicated suite

v17 -> v18:
* Fix lockdep_assert_held
* Fix spin_lock_init typo
* Fix CONFIG_TRACER_MAX_TRACE typo

v16 -> v17:
* Documentation and comments improvements.
* Create get/put_snapshot_map() for clearer code.
* Replace kzalloc with kcalloc.
* Fix -ENOMEM handling in rb_alloc_meta_page().
* Move flush(cpu_buffer->reader_page) behind the reader lock.
* Move all inc/dec of cpu_buffer->mapped behind reader lock and buffer
mutex. (removes READ_ONCE/WRITE_ONCE accesses).

v15 -> v16:
* Add comment for the dcache flush.
* Remove now unnecessary WRITE_ONCE for the meta-page.

v14 -> v15:
* Add meta-page and reader-page flush. Intends to fix the mapping
for VIVT and aliasing-VIPT data caches.
* -EPERM on VM_EXEC.
* Fix build warning !CONFIG_TRACER_MAX_TRACE.

v13 -> v14:
* All cpu_buffer->mapped readers use READ_ONCE (except for swap_cpu)
* on unmap, sync meta-page teardown with the reader_lock instead of
the synchronize_rcu.
* Add a dedicated spinlock for trace_array ->snapshot and ->mapped.
(intends to fix a lockdep issue)
* Add kerneldoc for flags and Reserved fields.
* Add kselftest for snapshot/map mutual exclusion.

v12 -> v13:
* Swap subbufs_{touched,lost} for Reserved fields.
* Add a flag field in the meta-page.
* Fix CONFIG_TRACER_MAX_TRACE.
* Rebase on top of trace/urgent.
* Add a comment for try_unregister_trigger()

v11 -> v12:
* Fix code sample mmap bug.
* Add logging in sample code.
* Reset tracer in selftest.
* Add a refcount for the snapshot users.
* Prevent mapping when there are snapshot users and vice versa.
* Refine the meta-page.
* Fix types in the meta-page.
* Collect Reviewed-by.

v10 -> v11:
* Add Documentation and code sample.
* Add a selftest.
* Move all the update to the meta-page into a single
rb_update_meta_page().
* rb_update_meta_page() is now called from
ring_buffer_map_get_reader() to fix NOBLOCK callers.
* kerneldoc for struct trace_meta_page.
* Add a patch to zero all the ring-buffer allocations.

v9 -> v10:
* Refactor rb_update_meta_page()
* In-loop declaration for foreach_subbuf_page()
* Check for cpu_buffer->mapped overflow

v8 -> v9:
* Fix the unlock path in ring_buffer_map()
* Fix cpu_buffer cast with rb_work_rq->is_cpu_buffer
* Rebase on linux-trace/for-next (3cb3091138ca0921c4569bcf7ffa062519639b6a)

v7 -> v8:
* Drop the subbufs renaming into bpages
* Use subbuf as a name when relevant

v6 -> v7:
* Rebase onto lore.kernel.org/lkml/20231215175502.106587604@goodmis.org/
* Support for subbufs
* Rename subbufs into bpages

v5 -> v6:
* Rebase on next-20230802.
* (unsigned long) -> (void *) cast for virt_to_page().
* Add a wait for the GET_READER_PAGE ioctl.
* Move writer fields update (overrun/pages_lost/entries/pages_touched)
in the irq_work.
* Rearrange id in struct buffer_page.
* Rearrange the meta-page.
* ring_buffer_meta_page -> trace_buffer_meta_page.
* Add meta_struct_len into the meta-page.

v4 -> v5:
* Trivial rebase onto 6.5-rc3 (previously 6.4-rc3)

v3 -> v4:
* Add to the meta-page:
- pages_lost / pages_read (allow to compute how full is the
ring-buffer)
- read (allow to compute how many entries can be read)
- A reader_page struct.
* Rename ring_buffer_meta_header -> ring_buffer_meta
* Rename ring_buffer_get_reader_page -> ring_buffer_map_get_reader_page
* Properly consume events on ring_buffer_map_get_reader_page() with
rb_advance_reader().

v2 -> v3:
* Remove data page list (for non-consuming read)
** Implies removing order > 0 meta-page
* Add a new meta page field ->read
* Rename ring_buffer_meta_page_header into ring_buffer_meta_header

v1 -> v2:
* Hide data_pages from the userspace struct
* Fix META_PAGE_MAX_PAGES
* Support for order > 0 meta-page
* Add missing page->mapping.

Vincent Donnefort (5):
ring-buffer: Allocate sub-buffers with __GFP_COMP
ring-buffer: Introducing ring-buffer mapping functions
tracing: Allow user-space mapping of the ring-buffer
Documentation: tracing: Add ring-buffer mapping
ring-buffer/selftest: Add ring-buffer mapping test

Documentation/trace/index.rst | 1 +
Documentation/trace/ring-buffer-map.rst | 106 +++++
include/linux/ring_buffer.h | 6 +
include/uapi/linux/trace_mmap.h | 48 ++
kernel/trace/ring_buffer.c | 415 +++++++++++++++++-
kernel/trace/trace.c | 104 ++++-
kernel/trace/trace.h | 1 +
.../testing/selftests/ring-buffer/.gitignore | 1 +
tools/testing/selftests/ring-buffer/Makefile | 8 +
tools/testing/selftests/ring-buffer/config | 2 +
.../testing/selftests/ring-buffer/map_test.c | 288 ++++++++++++
11 files changed, 969 insertions(+), 11 deletions(-)
create mode 100644 Documentation/trace/ring-buffer-map.rst
create mode 100644 include/uapi/linux/trace_mmap.h
create mode 100644 tools/testing/selftests/ring-buffer/.gitignore
create mode 100644 tools/testing/selftests/ring-buffer/Makefile
create mode 100644 tools/testing/selftests/ring-buffer/config
create mode 100644 tools/testing/selftests/ring-buffer/map_test.c

base-commit: 7604256cecef34a82333d9f78262d3180f4eb525
--
2.44.0.769.g3c40516874-goog