Re: [PATCH 1/5] perf/core: add AUX buffer ownership for kernel events

[sashiko-bot@kernel.org]

> Add an in-kernel AUX owner reference and setup/release helpers for
> kernel-created perf events that need an AUX buffer without a userspace
> mmap.  The setup path validates that the event is a kernel event with
> no parent, rejects non-power-of-two page counts and negative watermark
> values, allocates the perf buffer and AUX pages, records the kernel
> owner, and attaches the buffer under the same lock.
> 
> perf_event_release_aux() stops AUX writers, frees AUX storage, and
> detaches the buffer, in that order, matching the AUX teardown ordering
> of perf_mmap_close(): perf_pmu_output_stop() walks event->rb->event_list
> and rb_free_aux() must run while the buffer is still referenced by the
> event.  The single event->rb reference is dropped by
> ring_buffer_attach(event, NULL) itself, so the release path does not
> put it again.
> 
> Both functions run under event->mmap_mutex, the lock that already
> serialises ring-buffer attach/detach transitions for an event:
> perf_mmap(), perf_mmap_close(), _perf_event_set_output() and
> _free_event() all hold it around ring_buffer_attach().  Concurrent
> perf_event_release_aux() callers therefore go through the same
> serialisation point, and a second release call, or a release of a
> buffer this API does not own, is a no-op.
> 
> Keep aux_mmap_count dedicated to userspace mappings.
> perf_aux_output_begin() accepts a writer while either a userspace or kernel
> owner remains, preserving the existing teardown ordering.
> 
> Co-developed-by: Lian Wang <lianux.mm@gmail.com>
> Signed-off-by: Lian Wang <lianux.mm@gmail.com>
> Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261004173458.837842-1-kunwu.chan@gmail.com?part=1