linux-kernel - Re: [PATCHv3 03/24] mm: avoid PG

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Message-ID: <54DD08BC.2020008@redhat.com>
Date:	Thu, 12 Feb 2015 15:10:36 -0500
From:	Rik van Riel <riel@...hat.com>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
CC:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	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>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 03/24] mm: avoid PG_locked on tail pages

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Hash: SHA1

On 02/12/2015 02:55 PM, Rik van Riel wrote:
> On 02/12/2015 11:18 AM, Kirill A. Shutemov wrote:

>> @@ -490,6 +493,7 @@ extern int 
>> wait_on_page_bit_killable_timeout(struct page *page,
> 
>> static inline int wait_on_page_locked_killable(struct page *page)
>>  { +	page = compound_head(page); if (PageLocked(page)) return 
>> wait_on_page_bit_killable(page, PG_locked); return 0; @@ -510,6 
>> +514,7 @@ static inline void wake_up_page(struct page *page, int 
>> bit) */ static inline void wait_on_page_locked(struct page *page)
>>  { +	page = compound_head(page); if (PageLocked(page)) 
>> wait_on_page_bit(page, PG_locked); }
> 
> These are all atomic operations.
> 
> This may be a stupid question with the answer lurking somewhere in
> the other patches, but how do you ensure you operate on the right
> page lock during a THP collapse or split?

Kirill answered that question on IRC.

The VM takes a refcount on a page before attempting to take a page
lock, which prevents the THP code from doing anything with the
page. In other words, while we have a refcount on the page, we
will dereference the same page lock.

- -- 
All rights reversed
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