From: Steven Rostedt Date: Tue, 1 Apr 2025 20:25:54 +0000 (-0400) Subject: ring-buffer: Allow reserve_mem persistent ring buffers to be mmapped X-Git-Tag: v6.16-rc1~102^2~7 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c2a08311427cc8c5c547e5d700cb2f93d63fcb2a;p=thirdparty%2Fkernel%2Flinux.git ring-buffer: Allow reserve_mem persistent ring buffers to be mmapped When the persistent ring buffer is created from the memory returned by reserve_mem there is nothing prohibiting it to be memory mapped to user space. The memory is the same as the pages allocated by alloc_page(). The way the memory is managed by the ring buffer code is slightly different though and needs to be addressed. 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. A new rb_page_id() helper function is used to get and set the id depending on if the page is a normal memory allocated buffer or a physical memory mapped buffer. Cc: Linus Torvalds Cc: Masami Hiramatsu Cc: Mark Rutland Cc: Mathieu Desnoyers Cc: Andrew Morton Cc: Vincent Donnefort Cc: Vlastimil Babka Cc: Mike Rapoport Cc: Jann Horn Link: https://lore.kernel.org/20250401203332.246646011@goodmis.org Signed-off-by: Steven Rostedt (Google) --- diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 683aa57870fe5..e40f5c6d7908b 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -6003,6 +6003,39 @@ static void rb_clear_buffer_page(struct buffer_page *page) page->read = 0; } +/* + * When the buffer is memory mapped to user space, each sub buffer + * has a unique id that is used by the meta data to tell the user + * where the current reader page is. + * + * For a normal allocated ring buffer, the id is saved in the buffer page + * id field, and updated via this function. + * + * But for a fixed memory mapped buffer, the id is already assigned for + * fixed memory ording in the memory layout and can not be used. Instead + * the index of where the page lies in the memory layout is used. + * + * For the normal pages, set the buffer page id with the passed in @id + * value and return that. + * + * For fixed memory mapped pages, get the page index in the memory layout + * and return that as the id. + */ +static int rb_page_id(struct ring_buffer_per_cpu *cpu_buffer, + struct buffer_page *bpage, int id) +{ + /* + * For boot buffers, the id is the index, + * otherwise, set the buffer page with this id + */ + if (cpu_buffer->ring_meta) + id = rb_meta_subbuf_idx(cpu_buffer->ring_meta, bpage->page); + else + bpage->id = id; + + return id; +} + static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer) { struct trace_buffer_meta *meta = cpu_buffer->meta_page; @@ -6011,7 +6044,9 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer) return; meta->reader.read = cpu_buffer->reader_page->read; - meta->reader.id = cpu_buffer->reader_page->id; + meta->reader.id = rb_page_id(cpu_buffer, cpu_buffer->reader_page, + cpu_buffer->reader_page->id); + meta->reader.lost_events = cpu_buffer->lost_events; meta->entries = local_read(&cpu_buffer->entries); @@ -6927,23 +6962,29 @@ 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++; + id = rb_page_id(cpu_buffer, cpu_buffer->reader_page, id); + subbuf_ids[id++] = (unsigned long)cpu_buffer->reader_page->page; + cnt++; first_subbuf = subbuf = rb_set_head_page(cpu_buffer); do { + id = rb_page_id(cpu_buffer, subbuf, 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; diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 5b8db27fb6ef3..73d75cdb7e5be 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -8508,10 +8508,6 @@ static int tracing_buffers_mmap(struct file *filp, struct vm_area_struct *vma) if (iter->tr->flags & TRACE_ARRAY_FL_MEMMAP) return -ENODEV; - /* 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;