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

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Date:   Tue, 10 May 2022 16:58:13 -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 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.

thanks,

-- 
John Hubbard
NVIDIA