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Message-ID: <20121107123218.GY8218@suse.de>
Date: Wed, 7 Nov 2012 12:32:18 +0000
From: Mel Gorman <mgorman@...e.de>
To: Rik van Riel <riel@...hat.com>
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 Tue, Nov 06, 2012 at 02:18:18PM -0500, Rik van Riel wrote:
> 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.
Yeah, I leave the pte_numa in place until after the migration to avoid it.
> 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).
>
Well spotted, I'll fix it up.
> Furthermore, if migrate_misplaced_page moved the page,
> the !pte_same check will return false, and you will
> get a double fault.
>
Yes, you're right. autonuma avoids this problem by clearing _PAGE_NUMA
before the migration happens but then it will incur the TLB flush
overhead.
> 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?
>
I think I can do that without reaching too far into migrate.c by abusing
the migration callback handler to return the location of the new page.
I'll see what I can do.
> Also, we may want the change for ptep_clear_flush
> that avoids flushing remote TLBs for a pte without
> the _PAGE_PRESENT bit set.
>
Maybe but initially I'll limit it to try_to_unmap_one.
Thanks!
--
Mel Gorman
SUSE Labs
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