linux-kernel - Re: [PATCH, RFC 00/10] THP refcounting redesign

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Date:	Wed, 11 Jun 2014 01:14:31 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Christoph Lameter <cl@...two.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH, RFC 00/10] THP refcounting redesign

On Wed, Jun 11, 2014 at 12:04:51AM +0200, Andrea Arcangeli wrote:
> On Tue, Jun 10, 2014 at 11:46:40PM +0300, Kirill A. Shutemov wrote:
> > Agreed. The patchset drops tail page refcounting.
> 
> Very possibly I misread something or a later patch fixes this up, I
> just did a basic code review, but from the new code of split_huge_page
> it looks like it returns -EBUSY after checking the individual tail
> page refcounts, so it's not clear how that defines as "dropped".

page_mapcount() here is really mapcount: how many times the page is
mapped, not pins on tail pages as we have it now.

> 
> +       for (i = 0; i < HPAGE_PMD_NR; i++)
> +               tail_count += page_mapcount(page + i);
> +       if (tail_count != page_count(page) - 1) {
> +               BUG_ON(tail_count > page_count(page) - 1);
> +               compound_unlock(page);
> +               spin_unlock_irq(&zone->lru_lock);
> +               return -EBUSY;
> --
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-- 
 Kirill A. Shutemov
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