Re: [PATCH v20 0/5] Introducing trace buffer mapping by user-space
From: Steven Rostedt
Date: Wed Apr 10 2024 - 13:53:44 EST
Hi Andrew, et.al.
Linus said it's a hard requirement that this code gets an Acked-by (or
Reviewed-by) from the memory sub-maintainers before he will accept it.
He was upset that we faulted in pages one at a time instead of mapping it
in one go:
https://lore.kernel.org/all/CAHk-=wh5wWeib7+kVHpBVtUn7kx7GGadWqb5mW5FYTdewEfL=w@xxxxxxxxxxxxxx/
Could you take a look at patches 1-3 to make sure they look sane from a
memory management point of view?
I really want this applied in the next merge window.
Thanks!
-- Steve
On Sat, 6 Apr 2024 18:36:44 +0100
Vincent Donnefort <vdonnefort@xxxxxxxxxx> wrote:
> 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
>
> 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@xxxxxxxxxxx/
> * 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 | 403 +++++++++++++++++-
> kernel/trace/trace.c | 113 ++++-
> kernel/trace/trace.h | 1 +
> tools/testing/selftests/ring-buffer/Makefile | 8 +
> tools/testing/selftests/ring-buffer/config | 2 +
> .../testing/selftests/ring-buffer/map_test.c | 302 +++++++++++++
> 10 files changed, 979 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/Makefile
> create mode 100644 tools/testing/selftests/ring-buffer/config
> create mode 100644 tools/testing/selftests/ring-buffer/map_test.c
>
>
> base-commit: 7604256cecef34a82333d9f78262d3180f4eb525