Re: [PATCHv5 19/28] mm: store mapcount for compound page separately

[Vlastimil Babka]

On 04/23/2015 11:03 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.
>
> We use the same approach as with compound page destructor and compound
> order: use space in first tail page, ->mapping this time.
>
> page_mapcount() counts both: PTE and PMD mappings of the page.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> ---
>   include/linux/mm.h       | 25 ++++++++++++--
>   include/linux/mm_types.h |  1 +
>   include/linux/rmap.h     |  4 +--
>   mm/debug.c               |  5 ++-
>   mm/huge_memory.c         |  2 +-
>   mm/hugetlb.c             |  4 +--
>   mm/memory.c              |  2 +-
>   mm/migrate.c             |  2 +-
>   mm/page_alloc.c          | 14 ++++++--
>   mm/rmap.c                | 87 +++++++++++++++++++++++++++++++++++++-----------
>   10 files changed, 114 insertions(+), 32 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dad667d99304..33cb3aa647a6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -393,6 +393,19 @@ static inline int is_vmalloc_or_module_addr(const void *x)
>
>   extern void kvfree(const void *addr);
>
> +static inline atomic_t *compound_mapcount_ptr(struct page *page)
> +{
> +	return &page[1].compound_mapcount;
> +}
> +
> +static inline int compound_mapcount(struct page *page)
> +{
> +	if (!PageCompound(page))
> +		return 0;
> +	page = compound_head(page);
> +	return atomic_read(compound_mapcount_ptr(page)) + 1;
> +}
> +
>   /*
>    * The atomic page->_mapcount, starts from -1: so that transitions
>    * both from it and to it can be tracked, using atomic_inc_and_test

What's not shown here is the implementation of page_mapcount_reset() 
that's unchanged... is that correct from all callers?

> @@ -405,8 +418,16 @@ static inline void page_mapcount_reset(struct page *page)
>
>   static inline int page_mapcount(struct page *page)
>   {
> +	int ret;
>   	VM_BUG_ON_PAGE(PageSlab(page), page);
> -	return atomic_read(&page->_mapcount) + 1;
> +	ret = atomic_read(&page->_mapcount) + 1;
> +	/*
> +	 * Positive compound_mapcount() offsets ->_mapcount in every page by
> +	 * one. Let's substract it here.
> +	 */

This could use some more detailed explanation, or at least pointers to 
the relevant rmap functions. Also in commit message.

> +	if (compound_mapcount(page))
> +	       ret += compound_mapcount(page) - 1;

This looks like it could uselessly duplicate-inline the code for 
compound_mapcount(). It has atomics and smp_rmb() so I'm not sure if the 
compiler can just "squash it".

On the other hand, a simple atomic read that was page_mapcount() has 
turned into multiple atomic reads and flag checks. What about the 
stability of the whole result? Are all callers ok? (maybe a later page 
deals with it).

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