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Subject: Re: [PATCH v5 04/10] hugetlb/userfaultfd: Unshare all pmds for
 hugetlbfs when register wp
To:     Peter Xu <peterx@redhat.com>
Cc:     Axel Rasmussen <axelrasmussen@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
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        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
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        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
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        Mina Almasry <almasrymina@google.com>,
        Oliver Upton <oupton@google.com>
References: <20210210212200.1097784-1-axelrasmussen@google.com>
 <20210210212200.1097784-5-axelrasmussen@google.com>
 <517f3477-cb80-6dc9-bda0-b147dea68f95@oracle.com>
 <20210212211856.GD3171@xz-x1>
From:   Mike Kravetz <mike.kravetz@oracle.com>
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Date:   Fri, 12 Feb 2021 13:34:03 -0800
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On 2/12/21 1:18 PM, Peter Xu wrote:
> On Fri, Feb 12, 2021 at 10:11:39AM -0800, Mike Kravetz wrote:
>> On 2/10/21 1:21 PM, Axel Rasmussen wrote:
>>> From: Peter Xu <peterx@redhat.com>
>>> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
>>> index b8200782dede..ff50c8528113 100644
>>> --- a/include/linux/mmu_notifier.h
>>> +++ b/include/linux/mmu_notifier.h
>>> @@ -51,6 +51,7 @@ enum mmu_notifier_event {
>>>  	MMU_NOTIFY_SOFT_DIRTY,
>>>  	MMU_NOTIFY_RELEASE,
>>>  	MMU_NOTIFY_MIGRATE,
>>> +	MMU_NOTIFY_HUGETLB_UNSHARE,
>>
>> I don't claim to know much about mmu notifiers.  Currently, we use other
>> event notifiers such as MMU_NOTIFY_CLEAR.  I guess we do 'clear' page table
>> entries if we unshare.  More than happy to have a MMU_NOTIFY_HUGETLB_UNSHARE
>> event, but will consumers of the notifications know what this new event type
>> means?  And, if we introduce this should we use this other places where
>> huge_pmd_unshare is called?
> 
> Yeah AFAICT that is a new feature to mmu notifiers and it's not really used a
> lot by consumers yet.  Hmm... is there really any consumer at all? I simply
> grepped MMU_NOTIFY_UNMAP and see no hook took special care of that.  So it's
> some extra information that the upper layer would like to deliever to the
> notifiers, it's just not vastly used so far.
> 
> So far I didn't worry too much on that either.  MMU_NOTIFY_HUGETLB_UNSHARE is
> introduced here simply because I tried to make it explicit, then it's easy to
> be overwritten one day if we think huge pmd unshare is not worth a standalone
> flag but reuse some other common one.  But I think at least I owe one
> documentation of that new enum. :)
> 
> I'm not extremely willing to touch the rest callers of huge pmd unshare yet,
> unless I've a solid reason.  E.g., one day maybe one mmu notifier hook would
> start to monitor some events, then that's a better place imho to change them.
> Otherwise any of my future change could be vague, imho.
> 
> For this patch - how about I simply go back to use MMU_NOTIFIER_CLEAR instead?

I'm good with the new MMU_NOTIFY_HUGETLB_UNSHARE and agree with your reasoning
for adding it.  I really did not know enough about usage which caused me to
question.
-- 
Mike Kravetz