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Date: Thu, 12 Feb 2015 15:10:36 -0500
From: Rik van Riel <riel@redhat.com>
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>
CC: Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
        Vlastimil Babka <vbabka@suse.cz>, 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>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCHv3 03/24] mm: avoid PG_locked on tail pages
References: <1423757918-197669-1-git-send-email-kirill.shutemov@linux.intel.com> <1423757918-197669-4-git-send-email-kirill.shutemov@linux.intel.com> <54DD054E.7000605@redhat.com>
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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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