Re: [PATCH 06/19] mm: store mapcount for compound page separate

[Jerome Marchand]

On 11/19/2014 02:00 PM, Kirill A. Shutemov wrote:
> On Wed, Nov 19, 2014 at 11:51:09AM +0100, Jerome Marchand wrote:
>> On 11/05/2014 03:49 PM, Kirill A. Shutemov wrote:
>>> We're going to allow mapping of individual 4k pages of THP compound and
>>> we need a cheap way to find out how many time the compound page is
>>> mapped with PMD -- compound_mapcount() does this.
>>>
>>> page_mapcount() counts both: PTE and PMD mappings of the page.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> ---
>>>  include/linux/mm.h   | 17 +++++++++++++++--
>>>  include/linux/rmap.h |  4 ++--
>>>  mm/huge_memory.c     | 23 ++++++++++++++---------
>>>  mm/hugetlb.c         |  4 ++--
>>>  mm/memory.c          |  2 +-
>>>  mm/migrate.c         |  2 +-
>>>  mm/page_alloc.c      | 13 ++++++++++---
>>>  mm/rmap.c            | 50 +++++++++++++++++++++++++++++++++++++++++++-------
>>>  8 files changed, 88 insertions(+), 27 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 1825c468f158..aef03acff228 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -435,6 +435,19 @@ static inline struct page *compound_head(struct page *page)
>>>  	return page;
>>>  }
>>>  
>>> +static inline atomic_t *compound_mapcount_ptr(struct page *page)
>>> +{
>>> +	return (atomic_t *)&page[1].mapping;
>>> +}
>>
>> IIUC your patch overloads the unused mapping field of the first tail
>> page to store the PMD mapcount. That's a non obvious trick. Why not make
>> it more explicit by adding a new field (say compound_mapcount - and the
>> appropriate comment of course) to the union to which mapping already belong?
> 
> I don't think we want to bloat struct page description: nobody outside of
> helpers should use it direcly. And it's exactly what we did to store
> compound page destructor and compound page order.

Yes, but hiding it might make people think this field is unused when
it's not. If it has been done that way for a while, maybe it's not as
much trouble as I think it is, but could you at least add a comment in
the helper.

> 
>> The patch description would benefit from more explanation too.
> 
> Agreed.
>

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