Re: [PATCH RFC 05/10] mm/hugetlb: Make walk_hugetlb_range() RCU-safe

[Peter Xu <peterx@redhat.com>]

On Sun, Nov 06, 2022 at 04:14:10PM +0800, kernel test robot wrote:
> 
> Greeting,
> 
> FYI, we noticed WARNING:suspicious_RCU_usage due to commit (built with gcc-11):
> 
> commit: 8b7e3b7ca3897ebc4cb7b23c65a4618d64056e3b ("[PATCH RFC 05/10] mm/hugetlb: Make walk_hugetlb_range() RCU-safe")
> url: https://github.com/intel-lab-lkp/linux/commits/Peter-Xu/mm-hugetlb-Make-huge_pte_offset-thread-safe-for-pmd-unshare/20221031-053221
> base: https://git.kernel.org/cgit/linux/kernel/git/akpm/mm.git mm-everything
> patch link: https://lore.kernel.org/lkml/20221030212929.335473-6-peterx@redhat.com
> patch subject: [PATCH RFC 05/10] mm/hugetlb: Make walk_hugetlb_range() RCU-safe
> 
> in testcase: kernel-selftests
> version: kernel-selftests-x86_64-9313ba54-1_20221017
> with following parameters:
> 
> 	sc_nr_hugepages: 2
> 	group: vm
> 
> test-description: The kernel contains a set of "self tests" under the tools/testing/selftests/ directory. These are intended to be small unit tests to exercise individual code paths in the kernel.
> test-url: https://www.kernel.org/doc/Documentation/kselftest.txt
> 
> 
> on test machine: 12 threads 1 sockets Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (Coffee Lake) with 16G memory
> 
> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> 
> 
> If you fix the issue, kindly add following tag
> | Reported-by: kernel test robot <oliver.sang@intel.com>
> | Link: https://lore.kernel.org/oe-lkp/202211061521.28931f7-oliver.sang@intel.com
> 
> 
> kern  :warn  : [  181.942648] WARNING: suspicious RCU usage
> kern  :warn  : [  181.943175] 6.1.0-rc1-00309-g8b7e3b7ca389 #1 Tainted: G S
> kern  :warn  : [  181.943972] -----------------------------
> kern  :warn  : [  181.944526] include/linux/rcupdate.h:364 Illegal context switch in RCU read-side critical section!
> kern  :warn  : [  181.945559]
> other info that might help us debug this:
> 
> kern  :warn  : [  181.946625]
> rcu_scheduler_active = 2, debug_locks = 1
> kern  :warn  : [  181.947473] 2 locks held by hmm-tests/9934:
> kern :warn : [  181.948016] #0: ffff8884325b2d18 (&mm->mmap_lock#2){++++}-{3:3}, at: dmirror_fault (test_hmm.c:?) test_hmm
> kern :warn : [  181.949129] #1: ffffffff858a7860 (rcu_read_lock){....}-{1:2}, at: walk_hugetlb_range (pagewalk.c:?) 
> kern  :warn  : [  181.950161]
> stack backtrace:
> kern  :warn  : [  181.950780] CPU: 9 PID: 9934 Comm: hmm-tests Tainted: G S                 6.1.0-rc1-00309-g8b7e3b7ca389 #1
> kern  :warn  : [  181.951863] Hardware name: Dell Inc. Vostro 3670/0HVPDY, BIOS 1.5.11 12/24/2018
> kern  :warn  : [  181.952709] Call Trace:
> kern  :warn  : [  181.953070]  <TASK>
> kern :warn : [  181.953403] dump_stack_lvl (??:?) 
> kern :warn : [  181.953890] __might_resched (??:?) 
> kern :warn : [  181.954403] __mutex_lock (mutex.c:?) 
> kern :warn : [  181.954886] ? validate_chain (lockdep.c:?) 
> kern :warn : [  181.955405] ? hugetlb_fault (??:?) 
> kern :warn : [  181.955926] ? mark_lock+0xca/0xac0 
> kern :warn : [  181.956450] ? mutex_lock_io_nested (mutex.c:?) 
> kern :warn : [  181.957039] ? check_prev_add (lockdep.c:?) 
> kern :warn : [  181.957580] ? hugetlb_vm_op_pagesize (hugetlb.c:?) 
> kern :warn : [  181.958177] ? hugetlb_fault (??:?) 
> kern :warn : [  181.958690] hugetlb_fault (??:?) 
> kern :warn : [  181.959199] ? find_held_lock (lockdep.c:?) 
> kern :warn : [  181.959709] ? hugetlb_no_page (??:?) 
> kern :warn : [  181.960255] ? __lock_release (lockdep.c:?) 
> kern :warn : [  181.960772] ? lock_downgrade (lockdep.c:?) 
> kern :warn : [  181.961292] ? lock_is_held_type (??:?) 
> kern :warn : [  181.961830] ? handle_mm_fault (??:?) 
> kern :warn : [  181.962363] handle_mm_fault (??:?) 
> kern :warn : [  181.962870] ? hmm_vma_walk_hugetlb_entry (hmm.c:?) 
> kern :warn : [  181.963501] hmm_vma_fault (hmm.c:?) 
> kern :warn : [  181.964096] walk_hugetlb_range (pagewalk.c:?) 
> kern :warn : [  181.964639] __walk_page_range (pagewalk.c:?) 
> kern :warn : [  181.965160] walk_page_range (??:?) 
> kern :warn : [  181.965670] ? __walk_page_range (??:?) 
> kern :warn : [  181.966213] ? rcu_read_unlock (main.c:?) 
> kern :warn : [  181.966718] ? lock_is_held_type (??:?) 
> kern :warn : [  181.967259] ? mmu_interval_read_begin (??:?) 
> kern :warn : [  181.967855] ? lock_is_held_type (??:?) 
> kern :warn : [  181.968400] hmm_range_fault (??:?) 
> kern :warn : [  181.968911] ? down_read (??:?) 
> kern :warn : [  181.969383] ? hmm_vma_fault (??:?) 
> kern :warn : [  181.969891] ? __lock_release (lockdep.c:?) 
> kern :warn : [  181.970416] dmirror_fault (test_hmm.c:?) test_hmm
> kern :warn : [  181.971012] ? dmirror_migrate_to_system+0x590/0x590 test_hmm
> kern :warn : [  181.971847] ? find_held_lock (lockdep.c:?) 
> kern :warn : [  181.972355] ? dmirror_write+0x202/0x310 test_hmm
> kern :warn : [  181.973069] ? __lock_release (lockdep.c:?) 
> kern :warn : [  181.973586] ? lock_downgrade (lockdep.c:?) 
> kern :warn : [  181.974107] ? lock_is_held_type (??:?) 
> kern :warn : [  181.974641] ? dmirror_write+0x202/0x310 test_hmm
> kern :warn : [  181.975355] ? lock_release (??:?) 
> kern :warn : [  181.975845] ? __mutex_unlock_slowpath (mutex.c:?) 
> kern :warn : [  181.976444] ? bit_wait_io_timeout (mutex.c:?) 
> kern :warn : [  181.977008] ? lock_is_held_type (??:?) 
> kern :warn : [  181.977547] ? dmirror_do_write (test_hmm.c:?) test_hmm
> kern :warn : [  181.978185] dmirror_write+0x1bf/0x310 test_hmm
> kern :warn : [  181.978881] ? dmirror_fault (test_hmm.c:?) test_hmm
> kern :warn : [  181.979484] ? lock_is_held_type (??:?) 
> kern :warn : [  181.980021] ? __might_fault (??:?) 
> kern :warn : [  181.980523] ? lock_release (??:?) 
> kern :warn : [  181.981019] dmirror_fops_unlocked_ioctl (test_hmm.c:?) test_hmm
> kern :warn : [  181.981732] ? dmirror_exclusive+0x780/0x780 test_hmm
> kern :warn : [  181.982485] ? do_user_addr_fault (fault.c:?) 
> kern :warn : [  181.983042] ? __lock_release (lockdep.c:?) 
> kern :warn : [  181.983562] __x64_sys_ioctl (??:?) 
> kern :warn : [  181.984074] do_syscall_64 (??:?) 
> kern :warn : [  181.984545] ? do_user_addr_fault (fault.c:?) 
> kern :warn : [  181.985103] ? do_user_addr_fault (fault.c:?) 
> kern :warn : [  181.985654] ? irqentry_exit_to_user_mode (??:?) 
> kern :warn : [  181.986256] ? lockdep_hardirqs_on_prepare (lockdep.c:?) 
> kern :warn : [  181.986945] entry_SYSCALL_64_after_hwframe (??:?) 

So it is caused by the hmm code doing page fault during page walk, where
it'll go into the hugetlb fault logic and trying to take sleeptable locks..

That's slightly out of my expectation because logically I think the page
walk hooks should only do trivial works on the pte/pmd/.. being walked on,
rather than things as complicated as triggering a page fault as what HMM
does.  And it's also surprising to me that we can actually allow sleep.
But so far it looks safe.

Besides HMM it seems there's yet another user (enable_skey_walk_ops) that
can also yield itself by calling cond_resched().

My current plan is I may need to add some helpers so that when the hooks
decides to call code that can sleep, we need to notify the walker API.  It
could be something called walk_page_pause(), walk_page_cont(), then for
either a fault or cond_reched(), we could:

  walk_page_pause(&walk);
  hmm_vma_fault(); // or cond_reched(), etc.
  walk_page_cont(&walk);

We should probably also emphasize somewhere that mmap lock should never be
released for the whole page walk process, because walk_page_range() will
cache vma pointers.

If there's any better suggestion, please feel free to comment, or I'll give
it a shot with above approach in the next version.

-- 
Peter Xu