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