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SubjectRe: [PATCHv5 19/28] mm: store mapcount for compound page separately
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@linux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@oracle.com>
> ---
> 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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