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Subject[PATCH] perf: Disable page faults when getting phys address
Date
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@intel.com>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
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
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