linux-kernel - Re: [RFC PATCH] mm, oom: distinguish blockable mode for mmu notifiers

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Message-ID: <e7f3289e-3ee3-3f02-1947-9e4327e1a864@redhat.com>
Date:   Mon, 25 Jun 2018 10:10:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [RFC PATCH] mm, oom: distinguish blockable mode for mmu notifiers

On 25/06/2018 09:57, Michal Hocko wrote:
> On Sun 24-06-18 10:11:21, Paolo Bonzini wrote:
>> On 22/06/2018 17:02, Michal Hocko wrote:
>>> @@ -7215,6 +7216,8 @@ void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
>>>  	apic_address = gfn_to_hva(kvm, APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
>>>  	if (start <= apic_address && apic_address < end)
>>>  		kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD);
>>> +
>>> +	return 0;
>>
>> This is wrong, gfn_to_hva can sleep.
> 
> Hmm, I have tried to crawl the call chain and haven't found any
> sleepable locks taken. Maybe I am just missing something.
> __kvm_memslots has a complex locking assert. I do not see we would take
> slots_lock anywhere from the notifier call path. IIUC that means that
> users_count has to be zero at that time. I have no idea how that is
> guaranteed.

Nevermind, ENOCOFFEE.  This is gfn_to_hva, not gfn_to_pfn.  It only
needs SRCU.

Paolo