Messages in this thread | | | Date | Thu, 30 Apr 2020 12:16:20 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC][PATCH] x86/mm: Sync all vmalloc mappings before text_poke() |
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----- On Apr 30, 2020, at 12:11 PM, rostedt rostedt@goodmis.org wrote:
> On Thu, 30 Apr 2020 16:11:21 +0200 > Joerg Roedel <jroedel@suse.de> wrote: > >> Hi, >> >> On Wed, Apr 29, 2020 at 10:07:31AM -0400, Steven Rostedt wrote: >> > Talking with Mathieu about this on IRC, he pointed out that my code does >> > have a vzalloc() that is called: >> > >> > in trace_pid_write() >> > >> > pid_list->pids = vzalloc((pid_list->pid_max + 7) >> 3); >> > >> > This is done when -P1,2 is on the trace-cmd command line. >> >> Okay, tracked it down, some instrumentation in the page-fault and >> double-fault handler gave me the stack-traces. Here is what happens: >> >> As already pointed out, it all happens because of page-faults on the >> vzalloc'ed pid bitmap. It starts with this stack-trace: >> >> RIP: 0010:trace_event_ignore_this_pid+0x23/0x30 > > Interesting. Because that function is this: > > bool trace_event_ignore_this_pid(struct trace_event_file *trace_file) > { > struct trace_array *tr = trace_file->tr; > struct trace_array_cpu *data; > struct trace_pid_list *no_pid_list; > struct trace_pid_list *pid_list; > > pid_list = rcu_dereference_raw(tr->filtered_pids); > no_pid_list = rcu_dereference_raw(tr->filtered_no_pids); > > if (!pid_list && !no_pid_list) > return false; > > data = this_cpu_ptr(tr->array_buffer.data); > > return data->ignore_pid; > } > > Where it only sees if the pid masks exist. That is, it looks to see if > there's pointers to them, it doesn't actually touch the vmalloc'd area. > This check is to handle a race between allocating and deallocating the > buffers and setting the ignore_pid bit. The reading of these arrays is done > at sched_switch time, which sets or clears the ignore_pid field. > > That said, since this only happens on buffer instances (it does not trigger > on the top level instance, which uses the same code for the pid masks) > > Could this possibly be for the tr->array_buffer.data, which is allocated > with: > > allocate_trace_buffer() { > [..] > buf->data = alloc_percpu(struct trace_array_cpu); > > That is, the bug isn't the vmalloc being a problem, but perhaps the per_cpu > allocation. This would explain why this crashes with the buffer instance > and not with the top level instance. If it was related to the pid masks, > then it would trigger for either (because they act the same in allocating > at time of use). But when an instance is made, the tr->array_buffer.data is > created. Which for the top level happens at boot up and the pages would > have been synced long ago. But for a newly created instance, this happens > just before its used. This could possibly explain why it's not a problem > when doing it manually by hand, because the time between creating the > instance, and the time to start and stop the tracing, is long enough for > something to sync them page tables. > > tl;dr; It's not an issue with the vmalloc, it's an issue with per_cpu > allocations!
Did I mention that alloc_percpu uses:
static void *pcpu_mem_zalloc(size_t size, gfp_t gfp) { if (WARN_ON_ONCE(!slab_is_available())) return NULL;
if (size <= PAGE_SIZE) return kzalloc(size, gfp); else return __vmalloc(size, gfp | __GFP_ZERO, PAGE_KERNEL); }
So yeah, it's vmalloc'd memory when size > PAGE_SIZE.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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