Re: [PATCH v20 0/5] Introducing trace buffer mapping by user-space

From: Mike Rapoport
Date: Wed Apr 17 2024 - 00:56:41 EST


(added linux-mm)

On Wed, Apr 10, 2024 at 01:56:12PM -0400, Steven Rostedt wrote:
>
> 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
>

--
Sincerely yours,
Mike.