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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