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

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Message-ID: <20140610220451.GG19660@redhat.com>
Date:	Wed, 11 Jun 2014 00:04:51 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
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 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".

+       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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