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Date: Wed, 10 Apr 2024 13:56:12 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Andrew Morton <akpm@linux-foundation.org>, "Liam R. Howlett"
 <Liam.Howlett@oracle.com>, Vlastimil Babka <vbabka@suse.cz>, Lorenzo
 Stoakes <lstoakes@gmail.com>
Cc: Vincent Donnefort <vdonnefort@google.com>, mhiramat@kernel.org,
 linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
 mathieu.desnoyers@efficios.com, kernel-team@android.com,
 rdunlap@infradead.org
Subject: Re: [PATCH v20 0/5] Introducing trace buffer mapping by user-space
Message-ID: <20240410135612.5dc362e3@gandalf.local.home>
In-Reply-To: <20240406173649.3210836-1-vdonnefort@google.com>
References: <20240406173649.3210836-1-vdonnefort@google.com>
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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@mail.gmail.com/

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@google.com> 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@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                    | 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