linux-kernel - Re: [PATCH 13/19] mm: mempolicy: Use _PAGE

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Date:	Tue, 06 Nov 2012 14:18:18 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/19] mm: mempolicy: Use _PAGE_NUMA to migrate pages

On 11/06/2012 04:14 AM, Mel Gorman wrote:
> Note: Based on "mm/mpol: Use special PROT_NONE to migrate pages" but
> 	sufficiently different that the signed-off-bys were dropped
>
> Combine our previous _PAGE_NUMA, mpol_misplaced and migrate_misplaced_page()
> pieces into an effective migrate on fault scheme.
>
> Note that (on x86) we rely on PROT_NONE pages being !present and avoid
> the TLB flush from try_to_unmap(TTU_MIGRATION). This greatly improves the
> page-migration performance.
>
> Based-on-work-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Mel Gorman <mgorman@...e.de>


>   	page = vm_normal_page(vma, addr, pte);
>   	BUG_ON(!page);
> +
> +	get_page(page);
> +	current_nid = page_to_nid(page);
> +	target_nid = mpol_misplaced(page, vma, addr);
> +	if (target_nid == -1)
> +		goto clear_pmdnuma;
> +
> +	pte_unmap_unlock(ptep, ptl);
> +	migrate_misplaced_page(page, target_nid);
> +	page = NULL;
> +
> +	ptep = pte_offset_map_lock(mm, pmd, addr, &ptl);
> +	if (!pte_same(*ptep, pte))
> +		goto out_unlock;
> +

I see you tried to avoid the extraneous TLB flush
from inside migrate_misplaced_page. However,
try_to_unmap_one calls ptep_clear_flush, which will
currently still result in a remote TLB flush for
a _PAGE_NUMA pte, despite the pte not being
accessible for memory accesses (_PAGE_PRESENT not set).

Furthermore, if migrate_misplaced_page moved the page,
the !pte_same check will return false, and you will
get a double fault.

I wonder if migrate_misplaced_page should return a
struct page* or a pfn, so we can compute what "pte"
_should_ be, corrected for the new pfn, feed that
value to pte_same, and then avoid the double fault?

Also, we may want the change for ptep_clear_flush
that avoids flushing remote TLBs for a pte without
the _PAGE_PRESENT bit set.

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