linux-kernel - Re: [PATCHv2 04/28] mm: make remove_migration

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Message-ID: <56BE0E62.60806@intel.com>
Date:	Fri, 12 Feb 2016 08:54:58 -0800
From:	Dave Hansen <dave.hansen@...el.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Yang Shi <yang.shi@...aro.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 04/28] mm: make remove_migration_ptes() beyond
 mm/migration.c

On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote
> We also shouldn't try to mlock() pte-mapped huge pages: pte-mapeed THP
> pages are never mlocked.

That's kinda subtle.  Can you explain more?

If we did the following:

	ptr = mmap(NULL, 512*PAGE_SIZE, ...);
	mlock(ptr, 512*PAGE_SIZE);
	fork();
	munmap(ptr + 100 * PAGE_SIZE, PAGE_SIZE);

I'd expect to get two processes, each mapping the same compound THP, one
with a PMD and the other with 511 ptes and one hole.  Is there something
different that goes on?