linux-kernel - Re: [PATCH] KVM: x86: Do not create mmu_rmaps

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Message-ID: <eb3e95fc-0c05-f0f9-1801-92ec9b5f5845@amd.com>
Date:   Tue, 4 Jan 2022 16:08:14 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, vasant.hegde@....com,
        brijesh.singh@....com
Subject: Re: [PATCH] KVM: x86: Do not create mmu_rmaps_stat for TDP MMU

Hi Peter,

On 1/4/2022 3:19 PM, Peter Xu wrote:
> Hi, Nikunj,
> 
> On Tue, Jan 04, 2022 at 02:58:14PM +0530, Nikunj A Dadhania wrote:
>> With TDP MMU being the default now, access to mmu_rmaps_stat debugfs
>> file causes following oops:
>>
>> BUG: kernel NULL pointer dereference, address: 0000000000000000
>> PGD 0 P4D 0
>> Oops: 0000 [#1] PREEMPT SMP NOPTI
>> CPU: 7 PID: 3185 Comm: cat Not tainted 5.16.0-rc4+ #204
>> RIP: 0010:pte_list_count+0x6/0x40
>>  Call Trace:
>>   <TASK>
>>   ? kvm_mmu_rmaps_stat_show+0x15e/0x320
>>   seq_read_iter+0x126/0x4b0
>>   ? aa_file_perm+0x124/0x490
>>   seq_read+0xf5/0x140
>>   full_proxy_read+0x5c/0x80
>>   vfs_read+0x9f/0x1a0
>>   ksys_read+0x67/0xe0
>>   __x64_sys_read+0x19/0x20
>>   do_syscall_64+0x3b/0xc0
>>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>>  RIP: 0033:0x7fca6fc13912
>>
>> Create mmu_rmaps_stat debugfs file only when rmaps are created.
>>
>> Reported-by: Vasant Hegde <vasant.hegde@....com>
>> Tested-by: Vasant Hegde <vasant.hegde@....com>
>> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> 
> Shall we put the check into kvm_mmu_rmaps_stat_show()?  As iiuc the rmap can be
> allocated dynamically (shadow_root_allocated changing from 0->1).

Yes, that will cover the above case. And mmu_rmaps_stat file will be empty 
in case when rmaps are not allocated.

Regards
Nikunj