linux-kernel - Re: [PATCH 12/18] mm: numa: Defer TLB flush for THP migration as long as possible

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Date:	Mon, 09 Dec 2013 11:13:50 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Alex Thorlton <athorlton@....com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 12/18] mm: numa: Defer TLB flush for THP migration as
 long as possible

On 12/09/2013 02:09 AM, Mel Gorman wrote:

> diff --git a/mm/migrate.c b/mm/migrate.c
> index cfb4190..5372521 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1759,6 +1759,12 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>  		goto out_fail;
>  	}
>  
> +	/* PTL provides a memory barrier with change_protection_range */
> +	ptl = pmd_lock(mm, pmd);
> +	if (tlb_flush_pending(mm))
> +		flush_tlb_range(vma, mmun_start, mmun_end);
> +	spin_unlock(ptl);
> +
>  	/* Prepare a page as a migration target */
>  	__set_page_locked(new_page);
>  	SetPageSwapBacked(new_page);

I don't think there is a need for that extra memory barrier.

On the "set_tlb_flush_pending, turn ptes into NUMA ones" side, we
have a barrier in the form of the page table lock.

We only end up in this code path if the pte/pmd already is a NUMA
one, and we take several spinlocks along the way to doing this test.
That provides for the memory barrier in this code path.

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