linux-kernel - Re: [PATCH v4] mm: fix is_pinnable

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Message-ID: <54b5d177-f2f4-cef2-3a68-cd3b0b276f86@nvidia.com>
Date:   Wed, 11 May 2022 15:25:49 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        John Dias <joaodias@...gle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page

On 5/11/22 2:46 PM, Minchan Kim wrote:
>> I read that, but there was never any real justification there for needing
>> to prevent a re-read of mt, just a preference: "I'd like to keep use the local
>> variable mt's value in folloing conditions checks instead of refetching
>> the value from get_pageblock_migratetype."
>>
>> But I don't believe that there is any combination of values of mt that
>> will cause a problem here.
>>
>> I also think that once we pull in experts, they will tell us that the
>> compiler is not going to re-run a non-trivial function to re-fetch a
>> value, but I'm not one of those experts, so that's still arguable. But
>> imagine what the kernel code would look like if every time we call
>> a large function, we have to consider if it actually gets called some
>> arbitrary number of times, due to (anti-) optimizations by the compiler.
>> This seems like something that is not really happening.
> 
> Maybe, I might be paranoid since I have heard too subtle things
> about how compiler could changes high level language code so wanted
> be careful especially when we do lockless-stuff.
> 
> Who cares when we change the large(?) function to small(?) function
> later on? I'd like to hear from experts to decide it.
> 

Yes. But one thing that is still unanswered, that I think you can
answer, is: even if the compiler *did* re-read the mt variable, what
problems could that cause? I claim "no problems", because there is
no combination of 0, _CMA, _ISOLATE, _CMA|ISOLATE that will cause
problems here.

Any if that's true, then we can leave the experts alone, because
the answer is there without knowing what happens exactly to mt.

thanks,

-- 
John Hubbard
NVIDIA