Hello Al,
On Tue, 31 Jan 2023, Al Viro wrote:
m68k equivalent of 26178ec11ef3 "x86: mm: consolidate VM_FAULT_RETRY
handling" If e.g. get_user() triggers a page fault and a fatal signal is
caught, we might end up with handle_mm_fault() returning VM_FAULT_RETRY
and not doing anything to page tables. In such case we must *not*
return to the faulting insn - that would repeat the entire thing without
making any progress; what we need instead is to treat that as failed
(user) memory access.
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
That could be a bug I was chasing back in 2021 but never found. The mmap
stressors in stress-ng were triggering a crash on a Mac Quadras, though
only rarely. Sometimes it would run all day without a failure.
Last year when I started using GCC 12 to build the kernel, I saw the same
workload fail again but the failure mode had become a silent hang/livelock
instead of the oopses I got with GCC 6.
When I press the NMI button after the livelock I always see
do_page_fault() in the backtrace. So I've been testing your patch. I've
been running the same stress-ng reproducer for about 12 hours now with no
failures which looks promising.
In case that stress-ng testing is of use:
Tested-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
BTW, how did you identify that bug in do_page_fault()? If its the same bug
I was chasing, it could be an old one. The stress-ng logs I collected last
year include a crash from a v4.14 build.
---
arch/m68k/mm/fault.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 4d2837eb3e2a..228128e45c67 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -138,8 +138,11 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
fault = handle_mm_fault(vma, address, flags, regs);
pr_debug("handle_mm_fault returns %x\n", fault);
- if (fault_signal_pending(fault, regs))
+ if (fault_signal_pending(fault, regs)) {
+ if (!user_mode(regs))
+ goto no_context;
return 0;
+ }
/* The fault is fully completed (including releasing mmap lock) */
if (fault & VM_FAULT_COMPLETED)
