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

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Date:   Tue, 10 May 2022 17:09:37 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     John Hubbard <jhubbard@...dia.com>
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 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/