On Fri, 14 Feb 2025 07:07:12 -0500 Steven Rostedt <[email protected]> wrote:
> On Fri, 14 Feb 2025 16:13:32 +0900 > Masami Hiramatsu (Google) <[email protected]> wrote: > > > On Thu, 13 Feb 2025 21:21:47 -0500 > > Steven Rostedt <[email protected]> wrote: > > > > > On Fri, 14 Feb 2025 11:07:22 +0900 > > > Masami Hiramatsu (Google) <[email protected]> wrote: > > > > > > > This may be good for fixing crash short term but may not allow us to > > > > read the > > > > subbufs which is read right before crash. Can we also add an option to > > > > reset > > > > the read pointer in the previous boot for the persistent ring buffer? > > > > > > I'm not sure what you mean here. > > > > > > Note, this is just for mmapping the buffers. Currently we never tried it > > > as > > > if we did, it would have crashed. But this does not affect normal reads. > > > > But reading buffer via mmap() is already supported. Can we read all pages, > > which had been read by trace_pipe_raw in previous boot, again on this boot? > > It's not supported. If you try it, it will crash. This prevents reading via > mmap() on a boot buffer. I don't know what you are asking. Once this patch > is applied, mmap() will always fail on the boot buffer before or after you > start it. Hmm, I meant it is supported for other non-persisten ring buffer, isn't it? > > > > > Anyway, I made another patch to fix it to allow mmap() on persistent ring > > buffer here. I thought I can get the phys_addr from struct vm_area, but > > it could not for vmap()'d area. So this stores the phys_addr in > > trace_buffer. > > Which is way too complex and intrusive. If you want to allow mapping, all > it needs is this: Ah, vmalloc_to_page() works, I see. > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 07b421115692..9339adc88ad5 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -5927,12 +5941,18 @@ 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); > @@ -5940,7 +5960,12 @@ 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 > @@ -7041,7 +7066,10 @@ 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) > @@ -7187,6 +7215,7 @@ 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)) > @@ -7255,7 +7291,12 @@ 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); > > But this still doesn't work, as the index is still off (I'm still fixing it). Ah, OK. Thanks for fixing it. > > The persistent ring buffer uses the page->id for one thing but the user > space mmap() uses it for something else. Thanks you, > > -- Steve -- Masami Hiramatsu (Google) <[email protected]>
