On 5/7/19 12:55 PM, Yang Shi wrote:
On 5/7/19 10:19 AM, Jan Stancek wrote:----- 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: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 theHi,I'm seeing userspace program getting stuck on aarch64, on kernels 4.20and 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 notmoving forward - stdout output stops. Observed CPU usage is however100%.At this time, kernel appears to be busy handling page faults (~700k persecond): # 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 CPUsutilized 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: e8000805060f53I had traces in mmap() and munmap() as well, they don't get hit whenreproducer 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 causesall 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 issuesidentified in:https://lore.kernel.org/lkml/20190428212557.13482-2-longman@xxxxxxxxxx/vmas have been detached from the rbtree. So the mmap(), which is writer, in your test may steal the lock andexecute with the munmap(), which is the reader after the downgrade, inparallel 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 samevmas, 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 ptracewhich 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 countbefore increment Issue is still easily reproducible for me.I'm including output of mem_abort_decode() / show_pte() for sample PTE,thatI see in page fault loop. (I went through all bits, but couldn't findanything invalid about it) mem_abort_decode: Mem abort info: mem_abort_decode: ESR = 0x92000047mem_abort_decode: Exception class = DABT (lower EL), IL = 32 bitsmem_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 makingit 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:This makes sense. munmap() does free page tables, so "freed_tables" should be 1 instead of 0. So, in this case, __tlb_reset_range() should not be called.
Further investigation shows the __tlb_reset_range() call is added by commit a6d60245d6d9 ("asm-generic/tlb: Track which levels of the page tables have been cleared"), which is aimed to make TLB flush smarter for huge page.
The commit was merged by 4.20 too with my munmap optimization. So, it looks nobody noticed that munmap() could run into "force" TLB flush other than MADV_DONTNEED. And, it does remove page tables. So, I believe removing __tlb_reset_range() should be the right fix.
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);I'm still trying to understand this part. It's actually not smaller, but it changes:__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?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 readThat commit doesn't change anything about TLB flush, just move zappingtriggers: flush_tlb_all(); Any time reproducer stalls and I read debugfs file, it recovers immediately and resumes printing to stdout.pages under read mmap_sem as what MADV_DONTNEED does. I don't have aarch64 board to reproduce and debug it. And, I'm notfamiliar with aarch64 architecture either. But, some history told me the parallel zapping page may run into stale TLB and defer a flush meaningthat this call may observe pte_none and fails to flush the TLB. But,this has been solved by commit 56236a59556c ("mm: refactor TLB gatheringAPI") and 99baac21e458 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem").For more detail, please refer to commit 4647706ebeee ("mm: always flushVMA 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 loopThanksBut, 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 A3and 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^CI 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 2223 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
