Re: [PATCHv6 29/36] thp: implement split_huge_pmd()

From: Vlastimil Babka <vbabka@suse.cz>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Hugh Dickins <hughd@google.com>,
	Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
	Rik van Riel <riel@redhat.com>, Christoph Lameter <cl@gentwo.org>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Steve Capper <steve.capper@linaro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@suse.cz>,
	Jerome Marchand <jmarchan@redhat.com>,
	Sasha Levin <sasha.levin@oracle.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv6 29/36] thp: implement split_huge_pmd()
Date: Mon, 22 Jun 2015 18:01:33 +0200	[thread overview]
Message-ID: <5588315D.5070804@suse.cz> (raw)
In-Reply-To: <20150622111434.GC7934@node.dhcp.inet.fi>

On 06/22/2015 01:14 PM, Kirill A. Shutemov wrote:
> On Thu, Jun 11, 2015 at 11:49:48AM +0200, Vlastimil Babka wrote:
>> On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
>>
>> The order of actions here means that between TestSetPageDoubleMap() and the
>> atomic incs, anyone calling page_mapcount() on one of the pages not
>> processed by the for loop yet, will see a value lower by 1 from what he
>> should see. I wonder if that can cause any trouble somewhere, especially if
>> there's only one other compound mapping and page_mapcount() will return 0
>> instead of 1?
>
> Good catch. Thanks.
>
> What about this?
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f1f5731a893..cd0e6addb662 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2636,15 +2636,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                          for (i = 0; i < HPAGE_PMD_NR; i++)
>                                  atomic_dec(&page[i]._mapcount);
>                  }
> -       } else if (!TestSetPageDoubleMap(page)) {
> +       } else if (!PageDoubleMap(page)) {
>                  /*
>                   * The first PMD split for the compound page and we still
>                   * have other PMD mapping of the page: bump _mapcount in
>                   * every small page.
> +                *
>                   * This reference will go away with last compound_mapcount.
> +                *
> +                * Note, we need to increment mapcounts before setting
> +                * PG_double_map to avoid false-negative page_mapped().
>                   */
>                  for (i = 0; i < HPAGE_PMD_NR; i++)
>                          atomic_inc(&page[i]._mapcount);
> +
> +               if (TestSetPageDoubleMap(page)) {
> +                       /* Race with another  __split_huge_pmd() for the page */
> +                       for (i = 0; i < HPAGE_PMD_NR; i++)
> +                               atomic_dec(&page[i]._mapcount);
> +               }
>          }

Yeah that should work.

>          smp_wmb(); /* make pte visible before pmd */

>
>> Conversely, when clearing PageDoubleMap() above (or in one of those rmap
>> functions IIRC), one could see mapcount inflated by one. But I guess that's
>> less dangerous.
>
> I think it's safe.

OK.

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