Re: [bug] aarch64: userspace stalls on page fault after dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> 
> 
> On 5/7/19 4:51 AM, Jan Stancek wrote:
> > ----- Original Message -----
> >>
> >> On 5/6/19 2:35 PM, Jan Stancek wrote:
> >>> ----- Original Message -----
> >>>> On 5/5/19 7:10 AM, Jan Stancek wrote:
> >>>>> Hi,
> >>>>>
> >>>>> I'm seeing userspace program getting stuck on aarch64, on kernels 4.20
> >>>>> and
> >>>>> newer.
> >>>>> It stalls from seconds to hours.
> >>>>>
> >>>>> I have simplified it to following scenario (reproducer linked below
> >>>>> [1]):
> >>>>>      while (1):
> >>>>>        spawn Thread 1: mmap, write, munmap
> >>>>>        spawn Thread 2: <nothing>
> >>>>>
> >>>>> Thread 1 is sporadically getting stuck on write to mapped area.
> >>>>> User-space
> >>>>> is not
> >>>>> moving forward - stdout output stops. Observed CPU usage is however
> >>>>> 100%.
> >>>>>
> >>>>> At this time, kernel appears to be busy handling page faults (~700k per
> >>>>> second):
> >>>>>
> >>>>> # perf top -a -g
> >>>>> -   98.97%     8.30%  a.out                     [.] map_write_unmap
> >>>>>       - 23.52% map_write_unmap
> >>>>>          - 24.29% el0_sync
> >>>>>             - 10.42% do_mem_abort
> >>>>>                - 17.81% do_translation_fault
> >>>>>                   - 33.01% do_page_fault
> >>>>>                      - 56.18% handle_mm_fault
> >>>>>                           40.26% __handle_mm_fault
> >>>>>                           2.19% __ll_sc___cmpxchg_case_acq_4
> >>>>>                           0.87% mem_cgroup_from_task
> >>>>>                      - 6.18% find_vma
> >>>>>                           5.38% vmacache_find
> >>>>>                        1.35% __ll_sc___cmpxchg_case_acq_8
> >>>>>                        1.23% __ll_sc_atomic64_sub_return_release
> >>>>>                        0.78% down_read_trylock
> >>>>>               0.93% do_translation_fault
> >>>>>       + 8.30% thread_start
> >>>>>
> >>>>> #  perf stat -p 8189 -d
> >>>>> ^C
> >>>>>     Performance counter stats for process id '8189':
> >>>>>
> >>>>>            984.311350      task-clock (msec)         #    1.000 CPUs
> >>>>>            utilized
> >>>>>                     0      context-switches          #    0.000 K/sec
> >>>>>                     0      cpu-migrations            #    0.000 K/sec
> >>>>>               723,641      page-faults               #    0.735 M/sec
> >>>>>         2,559,199,434      cycles                    #    2.600 GHz
> >>>>>           711,933,112      instructions              #    0.28  insn
> >>>>>           per
> >>>>>           cycle
> >>>>>       <not supported>      branches
> >>>>>               757,658      branch-misses
> >>>>>           205,840,557      L1-dcache-loads           #  209.121 M/sec
> >>>>>            40,561,529      L1-dcache-load-misses     #   19.71% of all
> >>>>>            L1-dcache hits
> >>>>>       <not supported>      LLC-loads
> >>>>>       <not supported>      LLC-load-misses
> >>>>>
> >>>>>           0.984454892 seconds time elapsed
> >>>>>
> >>>>> With some extra traces, it appears looping in page fault for same
> >>>>> address,
> >>>>> over and over:
> >>>>>      do_page_fault // mm_flags: 0x55
> >>>>>        __do_page_fault
> >>>>>          __handle_mm_fault
> >>>>>            handle_pte_fault
> >>>>>              ptep_set_access_flags
> >>>>>                if (pte_same(pte, entry))  // pte: e8000805060f53,
> >>>>>                entry:
> >>>>>                e8000805060f53
> >>>>>
> >>>>> I had traces in mmap() and munmap() as well, they don't get hit when
> >>>>> reproducer
> >>>>> hits the bad state.
> >>>>>
> >>>>> Notes:
> >>>>> - I'm not able to reproduce this on x86.
> >>>>> - Attaching GDB or strace immediatelly recovers application from stall.
> >>>>> - It also seems to recover faster when system is busy with other tasks.
> >>>>> - MAP_SHARED vs. MAP_PRIVATE makes no difference.
> >>>>> - Turning off THP makes no difference.
> >>>>> - Reproducer [1] usually hits it within ~minute on HW described below.
> >>>>> - Longman mentioned that "When the rwsem becomes reader-owned, it
> >>>>> causes
> >>>>>      all the spinning writers to go to sleep adding wakeup latency to
> >>>>>      the time required to finish the critical sections", but this looks
> >>>>>      like busy loop, so I'm not sure if it's related to rwsem issues
> >>>>>      identified
> >>>>>      in:
> >>>>>      https://lore.kernel.org/lkml/20190428212557.13482-2-longman@redhat.com/
> >>>> It sounds possible to me. What the optimization done by the commit ("mm:
> >>>> mmap: zap pages with read mmap_sem in munmap") is to downgrade write
> >>>> rwsem to read when zapping pages and page table in munmap() after the
> >>>> vmas have been detached from the rbtree.
> >>>>
> >>>> So the mmap(), which is writer, in your test may steal the lock and
> >>>> execute with the munmap(), which is the reader after the downgrade, in
> >>>> parallel to break the mutual exclusion.
> >>>>
> >>>> In this case, the parallel mmap() may map to the same area since vmas
> >>>> have been detached by munmap(), then mmap() may create the complete same
> >>>> vmas, and page fault happens on the same vma at the same address.
> >>>>
> >>>> I'm not sure why gdb or strace could recover this, but they use ptrace
> >>>> which may acquire mmap_sem to break the parallel inadvertently.
> >>>>
> >>>> May you please try Waiman's patch to see if it makes any difference?
> >>> I don't see any difference in behaviour after applying:
> >>>     [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader count
> >>>     before increment
> >>> Issue is still easily reproducible for me.
> >>>
> >>> I'm including output of mem_abort_decode() / show_pte() for sample PTE,
> >>> that
> >>> I see in page fault loop. (I went through all bits, but couldn't find
> >>> anything invalid about it)
> >>>
> >>>     mem_abort_decode: Mem abort info:
> >>>     mem_abort_decode:   ESR = 0x92000047
> >>>     mem_abort_decode:   Exception class = DABT (lower EL), IL = 32 bits
> >>>     mem_abort_decode:   SET = 0, FnV = 0
> >>>     mem_abort_decode:   EA = 0, S1PTW = 0
> >>>     mem_abort_decode: Data abort info:
> >>>     mem_abort_decode:   ISV = 0, ISS = 0x00000047
> >>>     mem_abort_decode:   CM = 0, WnR = 1
> >>>     show_pte: user pgtable: 64k pages, 48-bit VAs, pgdp =
> >>>     0000000067027567
> >>>     show_pte: [0000ffff6dff0000] pgd=000000176bae0003
> >>>     show_pte: , pud=000000176bae0003
> >>>     show_pte: , pmd=000000174ad60003
> >>>     show_pte: , pte=00e80008023a0f53
> >>>     show_pte: , pte_pfn: 8023a
> >>>
> >>>     >>> print bin(0x47)
> >>>     0b1000111
> >>>
> >>>     Per D12-2779 (ARM Architecture Reference Manual),
> >>>         ISS encoding for an exception from an Instruction Abort:
> >>>       IFSC, bits [5:0], Instruction Fault Status Code
> >>>       0b000111 Translation fault, level 3
> >>>
> >>> ---
> >>>
> >>> My theory is that TLB is getting broken.
> > Theory continued:
> >
> > unmap_region() is batching updates to TLB (for vmas and page tables).
> > And at the same time another thread handles page fault for same mm,
> > which increases "tlb_flush_pending".
> >
> > tlb_finish_mmu() called from unmap_region() will thus set 'force = 1'.
> > And arch_tlb_finish_mmu() will in turn reset TLB range, presumably making
> > it smaller then it would be if force == 0.
> >
> > Change below appears to fix it:
> >
> > diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
> > index f2f03c655807..a4cef21bd62b 100644
> > --- a/mm/mmu_gather.c
> > +++ b/mm/mmu_gather.c
> > @@ -93,7 +93,7 @@ void arch_tlb_finish_mmu(struct mmu_gather *tlb,
> >          struct mmu_gather_batch *batch, *next;
> >   
> >          if (force) {
> > -               __tlb_reset_range(tlb);
> >                  __tlb_adjust_range(tlb, start, end - start);
> 
> I don't get why the change could fix it?

My guess is that reset clears "tlb->freed_tables", which changes how
tlb_flush() operates, see "bool last_level = !tlb->freed_tables;" in
arch/arm64/include/asm/tlb.h. Maybe that doesn't clear some intermediate
entries? No clue.

If I let it reset the range, but preserve "freed_tables", it also
seems to solve the problem:

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index f2f03c655807..17fb0d7edc03 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -93,8 +93,20 @@ void arch_tlb_finish_mmu(struct mmu_gather *tlb,
        struct mmu_gather_batch *batch, *next;
 
        if (force) {
-               __tlb_reset_range(tlb);
+               if (tlb->fullmm) {
+                       tlb->start = tlb->end = ~0;
+               } else {
+                       tlb->start = TASK_SIZE;
+                       tlb->end = 0;
+               }
                __tlb_adjust_range(tlb, start, end - start);

> __tlb_reset_range() just reset
> start and end to TASK_SIZE and 0, then __tlb_adjust_range() set proper
> start and end. I don't get why "force" flush smaller range?

I'm still trying to understand this part. It's actually not smaller, but it changes:

unmap_region()
  # vm_start: ffff49bd0000 vm_end: ffff49be0000
  ...
  # tlb.start, tlb.end: 1000000000000 0
  free_pgtables()
  # tlb.start, tlb.end: ffff40000000 ffff40010000
  tlb_finish_mmu()
    arch_tlb_finish_mmu()
      # will see force == 1
      # resets tlb.start, tlb.end to: ffff49bd0000 ffff49be0000

> 
> >          }
> >
> >>> I made a dummy kernel module that exports debugfs file, which on read
> >>> triggers:
> >>>     flush_tlb_all();
> >>>
> >>> Any time reproducer stalls and I read debugfs file, it recovers
> >>> immediately and resumes printing to stdout.
> >> That commit doesn't change anything about TLB flush, just move zapping
> >> pages under read mmap_sem as what MADV_DONTNEED does.
> >>
> >> I don't have aarch64 board to reproduce and debug it. And, I'm not
> >> familiar with aarch64 architecture either. But, some history told me the
> >> parallel zapping page may run into stale TLB and defer a flush meaning
> >> that this call may observe pte_none and fails to flush the TLB. But,
> >> this has been solved by commit 56236a59556c ("mm: refactor TLB gathering
> >> API") and 99baac21e458 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss
> >> problem").
> >>
> >> For more detail, please refer to commit 4647706ebeee ("mm: always flush
> >> VMA ranges affected by zap_page_range"). Copied Mel and Rik in this
> >> thread. Also added Will Deacon and Catalin Marinas, who are aarch64
> >> maintainers, in this loop
> > Thanks
> >
> >> But, your test (triggering TLB flush) does demonstrate TLB flush is
> >> *not* done properly at some point as expected for aarch64. Could you
> >> please give the below patch a try?
> > Your patch also fixes my reproducer.
> 
> Thanks for testing it.
> 
> >
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index ab650c2..ef41ad5 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -1336,8 +1336,10 @@ void unmap_vmas(struct mmu_gather *tlb,
> >>
> >>           mmu_notifier_range_init(&range, vma->vm_mm, start_addr,
> >>           end_addr);
> >>           mmu_notifier_invalidate_range_start(&range);
> >> -       for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next)
> >> +       for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) {
> >>                   unmap_single_vma(tlb, vma, start_addr, end_addr, NULL);
> >> +               flush_tlb_range(vma, start_addr, end_addr);
> >> +       }
> >>           mmu_notifier_invalidate_range_end(&range);
> >>    }
> >>
> >>>>> - I tried 2 different aarch64 systems so far: APM X-Gene CPU Potenza A3
> >>>>> and
> >>>>>      Qualcomm 65-LA-115-151.
> >>>>>      I can reproduce it on both with v5.1-rc7. It's easier to reproduce
> >>>>>      on latter one (for longer periods of time), which has 46 CPUs.
> >>>>> - Sample output of reproducer on otherwise idle system:
> >>>>>      # ./a.out
> >>>>>      [00000314] map_write_unmap took: 26305 ms
> >>>>>      [00000867] map_write_unmap took: 13642 ms
> >>>>>      [00002200] map_write_unmap took: 44237 ms
> >>>>>      [00002851] map_write_unmap took: 992 ms
> >>>>>      [00004725] map_write_unmap took: 542 ms
> >>>>>      [00006443] map_write_unmap took: 5333 ms
> >>>>>      [00006593] map_write_unmap took: 21162 ms
> >>>>>      [00007435] map_write_unmap took: 16982 ms
> >>>>>      [00007488] map_write unmap took: 13 ms^C
> >>>>>
> >>>>> I ran a bisect, which identified following commit as first bad one:
> >>>>>      dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
> >>>>>
> >>>>> I can also make the issue go away with following change:
> >>>>> diff --git a/mm/mmap.c b/mm/mmap.c
> >>>>> index 330f12c17fa1..13ce465740e2 100644
> >>>>> --- a/mm/mmap.c
> >>>>> +++ b/mm/mmap.c
> >>>>> @@ -2844,7 +2844,7 @@ EXPORT_SYMBOL(vm_munmap);
> >>>>>     SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
> >>>>>     {
> >>>>>            profile_munmap(addr);
> >>>>> -       return __vm_munmap(addr, len, true);
> >>>>> +       return __vm_munmap(addr, len, false);
> >>>>>     }
> >>>>>
> >>>>> # cat /proc/cpuinfo  | head
> >>>>> processor       : 0
> >>>>> BogoMIPS        : 40.00
> >>>>> Features        : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
> >>>>> asimdrdm
> >>>>> CPU implementer : 0x51
> >>>>> CPU architecture: 8
> >>>>> CPU variant     : 0x0
> >>>>> CPU part        : 0xc00
> >>>>> CPU revision    : 1
> >>>>>
> >>>>> # numactl -H
> >>>>> available: 1 nodes (0)
> >>>>> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> >>>>> 23
> >>>>> 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
> >>>>> node 0 size: 97938 MB
> >>>>> node 0 free: 95732 MB
> >>>>> node distances:
> >>>>> node   0
> >>>>>      0:  10
> >>>>>
> >>>>> Regards,
> >>>>> Jan
> >>>>>
> >>>>> [1]
> >>>>> https://github.com/jstancek/reproducers/blob/master/kernel/page_fault_stall/mmap5.c
> >>>>> [2]
> >>>>> https://github.com/jstancek/reproducers/blob/master/kernel/page_fault_stall/config
> >>
> 
>