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Date:   Wed, 11 May 2022 17:49:49 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        John Dias <joaodias@google.com>,
        David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH v4] mm: fix is_pinnable_page against on cma page
Message-ID: <20220512004949.GK1790663@paulmck-ThinkPad-P17-Gen-1>
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On Wed, May 11, 2022 at 05:34:52PM -0700, John Hubbard wrote:
> On 5/11/22 17:26, Minchan Kim wrote:
> > > > Let me try to say this more clearly: I don't think that the following
> > > > __READ_ONCE() statement can actually help anything, given that
> > > > get_pageblock_migratetype() is non-inlined:
> > > > 
> > > > +	int __mt = get_pageblock_migratetype(page);
> > > > +	int mt = __READ_ONCE(__mt);
> > > > +
> > > > +	if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
> > > > +		return false;
> > > > 
> > > > 
> > > > Am I missing anything here?
> > > 
> > > In the absence of future aggression from link-time optimizations (LTO),
> > > you are missing nothing.
> > 
> > A thing I want to note is Android kernel uses LTO full mode.
> 
> Thanks Paul for explaining the state of things.
> 
> Minchan, how about something like very close to your original draft,
> then, but with a little note, and the "&" as well:
> 
> int __mt = get_pageblock_migratetype(page);
> 
> /*
>  * Defend against future compiler LTO features, or code refactoring
>  * that inlines the above function, by forcing a single read. Because, this
>  * routine races with set_pageblock_migratetype(), and we want to avoid
>  * reading zero, when actually one or the other flags was set.
>  */
> int mt = __READ_ONCE(__mt);
> 
> if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
>     return false;
> 
> 
> ...which should make everyone comfortable and protected from the
> future sins of the compiler and linker teams? :)

This would work, but it would force a store to the stack and an immediate
reload.  Which might be OK on this code path.

But using READ_ONCE() in (I think?) __get_pfnblock_flags_mask()
would likely generate the same code that is produced today.

	word = READ_ONCE(bitmap[word_bitidx]);

But I could easily have missed a turn in that cascade of functions.  ;-)

Or there might be some code path that really hates a READ_ONCE() in
that place.

							Thanx, Paul