Re: [PATCHv3 03/24] mm: avoid PG_locked on tail pages

[Aneesh Kumar K.V]

Rik van Riel <riel@xxxxxxxxxx> writes:

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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.

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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