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

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Date:   Tue, 10 May 2022 21:32:05 -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 5/10/22 17:09, Minchan Kim wrote:
> On Tue, May 10, 2022 at 04:58:13PM -0700, John Hubbard wrote:
>> On 5/10/22 4:31 PM, Minchan Kim wrote:
>>>>> +	int __mt = get_pageblock_migratetype(page);
>>>>> +	int mt = __READ_ONCE(__mt);
>>>>
>>>> Although I saw the email discussion about this in v2, that discussion
>>>> didn't go far enough. It started with "don't use volatile", and went
>>>> on to "try __READ_ONCE() instead", but it should have continued on
>>>> to "you don't need this at all".
>>>
>>> That's really what I want to hear from experts so wanted to learn
>>> "Why". How could we prevent refetching of the mt if we don't use
>>> __READ_ONCE or volatile there?
>>>
>>>>
>>>> Because you don't. There is nothing you are racing with, and adding
>>>> __READ_ONCE() in order to avoid a completely not-going-to-happen
>>>> compiler re-invocation of a significant code block is just very wrong.
>>>>
>>>> So let's just let it go entirely. :)
>>>
>>> Yeah, once it's clear for everyone, I am happy to remove the
>>> unnecessary lines.
>>>
>>>>
>>>>> +
>>>>> +	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>>>>
>>
>> With or without __READ_ONCE() or volatile or anything else,
>> this code will do what you want. Which is: loosely check
>> for either of the above.
>>
>> What functional problem do you think you are preventing
>> with __READ_ONCE()? Because I don't see one.
> 
> I discussed the issue at v1 so please take a look.
> 
> https://lore.kernel.org/all/YnFvmc+eMoXvLCWf@google.com/

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.


thanks,
-- 
John Hubbard
NVIDIA