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Message-Id: <20250216002150.ef7dfcbc0b875dfebde31a5b@kernel.org>
Date: Sun, 16 Feb 2025 00:21:50 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
<linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
<mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Vincent Donnefort <vdonnefort@...gle.com>
Subject: Re: [PATCH] ring-buffer: Allow persistent ring buffers to be
mmapped
On Fri, 14 Feb 2025 13:04:42 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
>
> The persistent ring buffer uses vmap()'d memory to map the reserved memory
> from boot. But the user space mmap() to the ring buffer requires
> virt_to_page() to return a valid page. But that only works for core kernel
> addresses and not for vmap() addresses.
>
> If virt_addr_valid() fails on the page to be mapped, use vmalloc_to_page()
> instead.
>
> Also, the persistent memory uses the page->id for its own purpose where as
> the user mmap buffer currently uses that for the subbuf array mapped to
> user space. If the buffer is a persistent buffer, use the page index into
> that buffer as the identifier instead of the page->id.
>
> That is, the page->id for a persistent buffer, represents the order of the
> buffer is in the link list. ->id == 0 means it is the reader page.
> When a reader page is swapped, the new reader page's ->id gets zero, and
> the old reader page gets the ->id of the page that it swapped with.
>
> The user space mapping has the ->id is the index of where it was mapped in
> user space and does not change while it is mapped.
>
> Since the persistent buffer is fixed in its location, the index of where
> a page is in the memory range can be used as the "id" to put in the meta
> page array, and it can be mapped in the same order to user space as it is
> in the persistent memory.
Looks good and works for me.
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Tested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Thank you,
>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> kernel/trace/ring_buffer.c | 49 ++++++++++++++++++++++++++++++++------
> kernel/trace/trace.c | 4 ----
> 2 files changed, 42 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index bb6089c2951e..87caf9d48edb 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5950,12 +5950,18 @@ static void rb_clear_buffer_page(struct buffer_page *page)
> static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> {
> struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> + struct page *page;
>
> if (!meta)
> return;
>
> meta->reader.read = cpu_buffer->reader_page->read;
> - meta->reader.id = cpu_buffer->reader_page->id;
> + /* For boot buffers, the id is the index */
> + if (cpu_buffer->ring_meta)
> + meta->reader.id = rb_meta_subbuf_idx(cpu_buffer->ring_meta,
> + cpu_buffer->reader_page->page);
> + else
> + meta->reader.id = cpu_buffer->reader_page->id;
> meta->reader.lost_events = cpu_buffer->lost_events;
>
> meta->entries = local_read(&cpu_buffer->entries);
> @@ -5963,7 +5969,12 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> meta->read = cpu_buffer->read;
>
> /* Some archs do not have data cache coherency between kernel and user-space */
> - flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));
> + if (virt_addr_valid(cpu_buffer->meta_page))
> + page = virt_to_page(cpu_buffer->meta_page);
> + else
> + page = vmalloc_to_page(cpu_buffer->meta_page);
> +
> + flush_dcache_folio(page_folio(page));
> }
>
> static void
> @@ -6883,23 +6894,38 @@ static void rb_setup_ids_meta_page(struct ring_buffer_per_cpu *cpu_buffer,
> struct trace_buffer_meta *meta = cpu_buffer->meta_page;
> unsigned int nr_subbufs = cpu_buffer->nr_pages + 1;
> struct buffer_page *first_subbuf, *subbuf;
> + int cnt = 0;
> int id = 0;
>
> - subbuf_ids[id] = (unsigned long)cpu_buffer->reader_page->page;
> - cpu_buffer->reader_page->id = id++;
> + if (cpu_buffer->ring_meta)
> + id = rb_meta_subbuf_idx(cpu_buffer->ring_meta,
> + cpu_buffer->reader_page->page);
> + else
> + cpu_buffer->reader_page->id = id;
> +
> + subbuf_ids[id++] = (unsigned long)cpu_buffer->reader_page->page;
> + cnt++;
>
> first_subbuf = subbuf = rb_set_head_page(cpu_buffer);
> do {
> + if (cpu_buffer->ring_meta)
> + id = rb_meta_subbuf_idx(cpu_buffer->ring_meta,
> + subbuf->page);
> + else
> + subbuf->id = id;
> +
> if (WARN_ON(id >= nr_subbufs))
> break;
>
> subbuf_ids[id] = (unsigned long)subbuf->page;
> - subbuf->id = id;
>
> rb_inc_page(&subbuf);
> id++;
> + cnt++;
> } while (subbuf != first_subbuf);
>
> + WARN_ON(cnt != nr_subbufs);
> +
> /* install subbuf ID to kern VA translation */
> cpu_buffer->subbuf_ids = subbuf_ids;
>
> @@ -7064,7 +7090,10 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
> goto out;
> }
>
> - page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
> + if (virt_addr_valid(cpu_buffer->subbuf_ids[s]))
> + page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
> + else
> + page = vmalloc_to_page((void *)cpu_buffer->subbuf_ids[s]);
>
> for (; off < (1 << (subbuf_order)); off++, page++) {
> if (p >= nr_pages)
> @@ -7210,6 +7239,7 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
> unsigned long missed_events;
> unsigned long reader_size;
> unsigned long flags;
> + struct page *page;
>
> cpu_buffer = rb_get_mapped_buffer(buffer, cpu);
> if (IS_ERR(cpu_buffer))
> @@ -7278,7 +7308,12 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
>
> out:
> /* Some archs do not have data cache coherency between kernel and user-space */
> - flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));
> + if (virt_addr_valid(cpu_buffer->meta_page))
> + page = virt_to_page(cpu_buffer->meta_page);
> + else
> + page = vmalloc_to_page(cpu_buffer->meta_page);
> +
> + flush_dcache_folio(page_folio(page));
>
> rb_update_meta_page(cpu_buffer);
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 0e6d517e74e0..25ff37aab00f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -8279,10 +8279,6 @@ static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma)
> struct trace_iterator *iter = &info->iter;
> int ret = 0;
>
> - /* Currently the boot mapped buffer is not supported for mmap */
> - if (iter->tr->flags & TRACE_ARRAY_FL_BOOT)
> - return -ENODEV;
> -
> ret = get_snapshot_map(iter->tr);
> if (ret)
> return ret;
> --
> 2.47.2
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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