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

From: Andrea Arcangeli <aarcange@redhat.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Christoph Lameter <cl@gentwo.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@intel.com>,
	Hugh Dickins <hughd@google.com>, Mel Gorman <mgorman@suse.de>,
	Rik van Riel <riel@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH, RFC 00/10] THP refcounting redesign
Date: Wed, 11 Jun 2014 00:04:51 +0200	[thread overview]
Message-ID: <20140610220451.GG19660@redhat.com> (raw)
In-Reply-To: <20140610204640.GA9594@node.dhcp.inet.fi>

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;