linux-kernel - Re: [PATCHv5 07/28] thp, mlock: do not allow huge pages in mlocked area

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Date:	Tue, 19 May 2015 16:37:25 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 07/28] thp, mlock: do not allow huge pages in mlocked
 area

On 05/15/2015 03:41 PM, Kirill A. Shutemov wrote:
> On Fri, May 15, 2015 at 02:56:42PM +0200, Vlastimil Babka wrote:
>> On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
>>> With new refcounting THP can belong to several VMAs. This makes tricky
>>> to track THP pages, when they partially mlocked. It can lead to leaking
>>> mlocked pages to non-VM_LOCKED vmas and other problems.
>>> With this patch we will split all pages on mlock and avoid
>>> fault-in/collapse new THP in VM_LOCKED vmas.
>>>
>>> I've tried alternative approach: do not mark THP pages mlocked and keep
>>> them on normal LRUs. This way vmscan could try to split huge pages on
>>> memory pressure and free up subpages which doesn't belong to VM_LOCKED
>>> vmas.  But this is user-visible change: we screw up Mlocked accouting
>>> reported in meminfo, so I had to leave this approach aside.
>>>
>>> We can bring something better later, but this should be good enough for
>>> now.
>>
>> I can imagine people won't be happy about losing benefits of THP's when they
>> mlock().
>> How difficult would it be to support mlocked THP pages without splitting
>> until something actually tries to do a partial (un)mapping, and only then do
>> the split? That will support the most common case, no?
>
> Yes, it will.
>
> But what will we do if we fail to split huge page on munmap()? Fail
> munmap() with -EBUSY?

We could just unmlock the whole THP page and if we could make the 
deferred split done ASAP, and not waiting for memory pressure, the 
window with NR_MLOCK being undercounted would be minimized. Since the 
RLIMIT_MEMLOCK is tracked independently from NR_MLOCK, there should be 
no danger wrt breaching the limit due to undercounting here?


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