Date:   Wed, 11 Dec 2019 17:11:55 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2] hugetlbfs: Disable softIRQ when taking hugetlb_lock

On 12/11/19 2:19 PM, Waiman Long wrote:
> On 12/11/19 5:04 PM, Mike Kravetz wrote:
>> Cc: Michal
>>
>> Sorry for the late reply on this effort.
>>
>> On 12/11/19 11:46 AM, Waiman Long wrote:
>>> The following lockdep splat was observed when a certain hugetlbfs test
>>> was run:
>>>
>>> [  612.388273] ================================
>>> [  612.411273] WARNING: inconsistent lock state
>>> [  612.432273] 4.18.0-159.el8.x86_64+debug #1 Tainted: G        W --------- -  -
>>> [  612.469273] --------------------------------
>>> [  612.489273] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
>>> [  612.517273] swapper/30/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
>>> [  612.541273] ffffffff9acdc038 (hugetlb_lock){+.?.}, at: free_huge_page+0x36f/0xaa0
>>> [  612.576273] {SOFTIRQ-ON-W} state was registered at:
>>> [  612.598273]   lock_acquire+0x14f/0x3b0
>>> [  612.616273]   _raw_spin_lock+0x30/0x70
>>> [  612.634273]   __nr_hugepages_store_common+0x11b/0xb30
>>> [  612.657273]   hugetlb_sysctl_handler_common+0x209/0x2d0
>>> [  612.681273]   proc_sys_call_handler+0x37f/0x450
>>> [  612.703273]   vfs_write+0x157/0x460
>>> [  612.719273]   ksys_write+0xb8/0x170
>>> [  612.736273]   do_syscall_64+0xa5/0x4d0
>>> [  612.753273]   entry_SYSCALL_64_after_hwframe+0x6a/0xdf
>>> [  612.777273] irq event stamp: 691296
>>> [  612.794273] hardirqs last  enabled at (691296): [<ffffffff99bb034b>] _raw_spin_unlock_irqrestore+0x4b/0x60
>>> [  612.839273] hardirqs last disabled at (691295): [<ffffffff99bb0ad2>] _raw_spin_lock_irqsave+0x22/0x81
>>> [  612.882273] softirqs last  enabled at (691284): [<ffffffff97ff0c63>] irq_enter+0xc3/0xe0
>>> [  612.922273] softirqs last disabled at (691285): [<ffffffff97ff0ebe>] irq_exit+0x23e/0x2b0
>>> [  612.962273]
>>> [  612.962273] other info that might help us debug this:
>>> [  612.993273]  Possible unsafe locking scenario:
>>> [  612.993273]
>>> [  613.020273]        CPU0
>>> [  613.031273]        ----
>>> [  613.042273]   lock(hugetlb_lock);
>>> [  613.057273]   <Interrupt>
>>> [  613.069273]     lock(hugetlb_lock);
>>> [  613.085273]
>>> [  613.085273]  *** DEADLOCK ***
>>>       :
>>> [  613.245273] Call Trace:
>>> [  613.256273]  <IRQ>
>>> [  613.265273]  dump_stack+0x9a/0xf0
>>> [  613.281273]  mark_lock+0xd0c/0x12f0
>>> [  613.297273]  ? print_shortest_lock_dependencies+0x80/0x80
>>> [  613.322273]  ? sched_clock_cpu+0x18/0x1e0
>>> [  613.341273]  __lock_acquire+0x146b/0x48c0
>>> [  613.360273]  ? trace_hardirqs_on+0x10/0x10
>>> [  613.379273]  ? trace_hardirqs_on_caller+0x27b/0x580
>>> [  613.401273]  lock_acquire+0x14f/0x3b0
>>> [  613.419273]  ? free_huge_page+0x36f/0xaa0
>>> [  613.440273]  _raw_spin_lock+0x30/0x70
>>> [  613.458273]  ? free_huge_page+0x36f/0xaa0
>>> [  613.477273]  free_huge_page+0x36f/0xaa0
>>> [  613.495273]  bio_check_pages_dirty+0x2fc/0x5c0
>>> [  613.516273]  clone_endio+0x17f/0x670 [dm_mod]
>>> [  613.536273]  ? disable_discard+0x90/0x90 [dm_mod]
>>> [  613.558273]  ? bio_endio+0x4ba/0x930
>>> [  613.575273]  ? blk_account_io_completion+0x400/0x530
>>> [  613.598273]  blk_update_request+0x276/0xe50
>>> [  613.617273]  scsi_end_request+0x7b/0x6a0
>>> [  613.636273]  ? lock_downgrade+0x6f0/0x6f0
>>> [  613.654273]  scsi_io_completion+0x1c6/0x1570
>>> [  613.674273]  ? sd_completed_bytes+0x3a0/0x3a0 [sd_mod]
>>> [  613.698273]  ? scsi_mq_requeue_cmd+0xc0/0xc0
>>> [  613.718273]  blk_done_softirq+0x22e/0x350
>>> [  613.737273]  ? blk_softirq_cpu_dead+0x230/0x230
>>> [  613.758273]  __do_softirq+0x23d/0xad8
>>> [  613.776273]  irq_exit+0x23e/0x2b0
>>> [  613.792273]  do_IRQ+0x11a/0x200
>>> [  613.806273]  common_interrupt+0xf/0xf
>>> [  613.823273]  </IRQ>
>> This is interesting.  I'm trying to wrap my head around how we ended up
>> with a BIO pointing to a hugetlbfs page.  My 'guess' is that user space
>> code passed an address to some system call or driver.  And, that system
>> call or driver set up the IO.  For the purpose of addressing this issue,
>> it does not matter.  I am just a little confused/curious.
>>
>>> Since hugetlb_lock can be taken from both process and softIRQ contexts,
>>> we need to protect the lock from nested locking by disabling softIRQ
>>> using spin_lock_bh() before taking it.
>>>
>>> Currently, only free_huge_page() is known to be called from softIRQ
>>> context.
>> We discussed this exact same issue more than a year ago.  See,
>> https://lkml.org/lkml/2018/9/5/398
>>
>> At that time, the only 'known' caller of put_page for a hugetlbfs page from
>> softirq context was in powerpc specific code.  IIRC, Aneesh addressed the
>> issue last year by modifying the powerpc specific code.  The more general
>> issue in the hugetlbfs code was never addressed. :(
>>
>> As part of the discussion in the previous e-mail thread, the issue of
>> whether we should address put_page for hugetlbfs pages for only softirq
>> or extend to hardirq context was discussed.  The conclusion (or at least
>> suggestion from Andrew and Michal) was that we should modify code to allow
>> for calls from hardirq context.  The reasoning IIRC, was that put_page of
>> other pages was allowed from hardirq context, so hugetlbfs pages should be
>> no different.
>>
>> Matthew, do you think that reasoning from last year is still valid?  Should
>> we be targeting soft or hard irq calls?
>>
>> One other thing.  free_huge_page may also take a subpool specific lock via
>> spin_lock().  See hugepage_subpool_put_pages.  This would also need to take
>> irq context into account.
> 
> Thanks for the background information.
> 
> We will need to use spin_lock_irq() or spin_lock_irqsave() for allowing
> hardirq context calls like what is in the v1 patch. I will look further
> into the subpool specific lock also.

Sorry,

I did not fully read all of Matthew's comments/suggestions on the original
patch.  His initial suggestion was for a workqueue approach that you did
start implementing, but thought was too complex.  Andi also suggested this
approach.

The workqueue approach would address both soft and hard irq context issues.
As a result, I too think this is the approach we should explore.  Since there
is more than one lock involved, this also is reason for a work queue approach.

I'll take a look at initial workqueue implementation.  However, I have not
dealt with workqueues in some time so it may take few days to evaluate.
-- 
Mike Kravetz

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