linux-kernel - Re: [PATCH V3 1/3] arm64/mm: Drop __HAVE_ARCH_HUGE_PTEP

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Message-ID: <7642f615-23ff-56c7-25da-f84c409449b5@oracle.com>
Date:   Mon, 11 May 2020 11:49:52 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/3] arm64/mm: Drop __HAVE_ARCH_HUGE_PTEP_GET

On 5/10/20 9:02 PM, Anshuman Khandual wrote:
> On 05/09/2020 03:39 AM, Mike Kravetz wrote:
>> On 5/7/20 8:07 PM, Anshuman Khandual wrote:
>> I know you made this change in response to Will's comment.  And, since
>> changes were made to consistently use READ_ONCE in arm64 code, it makes
>> sense for that architecture.
>>
>> However, with this change to generic code, you introduce READ_ONCE to
>> other architectures where it was not used before.  Could this possibly
>> introduce inconsistencies in their use of READ_ONCE?  To be honest, I
>> am not very good at identifying any possible issues this could cause.
>> However, it does seem possible.
> 
> Could you please give some more details. Is there any particular problem
> which might be caused by this new READ_ONCE() here, that you you are
> concerned about. READ_ONCE() is already getting used in multiple places
> in core MM which can not be configured out (like mm/gup.c). It is getting
> used in core HugeTLB (mm/hugetlb.c) as well. AFAICS, there is no standard
> for using READ_ONCE() while walking page tables entries. We have examples
> in core MM for both ways.
> 
>>
>> Will was nervous about dropping this from arm64.  I'm just a little nervous
>> about adding it to other architectures.
>>
> AFAICS, __HAVE_ARCH_HUGE_PTEP_GET should be used on a platform only when
> a HugeTLB entry could not constructed by dereferencing a page table entry
> as in the case with ARM (32 bit). Using READ_ONCE() while dereferencing is
> really not a special case that will need __HAVE_ARCH_HUGE_PTEP_GET. Moving
> READ_ONCE() into generic definition solves the problem while also taking
> care of a known problem on arm64. IMHO, it seems like the right thing to
> do unless there is another problem that pops up some where else because of
> READ_ONCE().

I have no specific concerns or objection.  After thinking about it some more,
I do not see how adding the READ_ONCE could cause any issues.

Acked-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz