[PATCH] perf: Disable page faults when getting phys address

[Jiri Olsa]

We hit following warning when running tests on kernel
compiled with CONFIG_DEBUG_ATOMIC_SLEEP:

 WARNING: CPU: 19 PID: 4472 at mm/gup.c:2381 __get_user_pages_fast+0x1a4/0x200
 CPU: 19 PID: 4472 Comm: dummy Not tainted 5.6.0-rc6+ #3
 RIP: 0010:__get_user_pages_fast+0x1a4/0x200
 ...
 Call Trace:
  perf_prepare_sample+0xff1/0x1d90
  perf_event_output_forward+0xe8/0x210
  __perf_event_overflow+0x11a/0x310
  __intel_pmu_pebs_event+0x657/0x850
  intel_pmu_drain_pebs_nhm+0x7de/0x11d0
  handle_pmi_common+0x1b2/0x650
  intel_pmu_handle_irq+0x17b/0x370
  perf_event_nmi_handler+0x40/0x60
  nmi_handle+0x192/0x590
  default_do_nmi+0x6d/0x150
  do_nmi+0x2f9/0x3c0
  nmi+0x8e/0xd7

While __get_user_pages_fast is IRQ-safe it calls access_ok,
which warns on:

  WARN_ON_ONCE(!in_task() && !pagefault_disabled())

Peter suggested disabling page faults around __get_user_pages_fast,
which gets rid of the warning in access_ok call.

Cc: Kan Liang <kan.liang@xxxxxxxxx>
Cc: Madhavan Srinivasan <maddy@xxxxxxxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 kernel/events/core.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1569979c8912..ccbcb9049c70 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6916,9 +6916,12 @@ static u64 perf_virt_to_phys(u64 virt)
 		 * Try IRQ-safe __get_user_pages_fast first.
 		 * If failed, leave phys_addr as 0.
 		 */
-		if ((current->mm != NULL) &&
-		    (__get_user_pages_fast(virt, 1, 0, &p) == 1))
-			phys_addr = page_to_phys(p) + virt % PAGE_SIZE;
+		if (current->mm != NULL) {
+			pagefault_disable();
+			if (__get_user_pages_fast(virt, 1, 0, &p) == 1)
+				phys_addr = page_to_phys(p) + virt % PAGE_SIZE;
+			pagefault_enable();
+		}
 
 		if (p)
 			put_page(p);
-- 
2.18.2