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

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Date:	Tue, 03 Mar 2015 18:51:11 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.com>,
	"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>,
	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

Rik van Riel <riel@...hat.com> writes:

> -----BEGIN PGP SIGNED MESSAGE-----
> 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.

Can we explain this more ? Don't we allow a thp split to happen even if
we have page refcount ?.

-aneesh

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