[v6,03/12] mm: introduce page_ref_sub_return()
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Message ID 20200211001536.1027652-4-jhubbard@nvidia.com
State New
Headers show
  • mm/gup: track FOLL_PIN pages
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Commit Message

John Hubbard Feb. 11, 2020, 12:15 a.m. UTC
An upcoming patch requires subtracting a large chunk of refcounts from
a page, and checking what the resulting refcount is. This is a little
different than the usual "check for zero refcount" that many of the
page ref functions already do. However, it is similar to a few other
routines that (like this one) are generally useful for things such as
1-based refcounting.

Add page_ref_sub_return(), that subtracts a chunk of refcounts
atomically, and returns an atomic snapshot of the result.

Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
 include/linux/page_ref.h | 9 +++++++++
 1 file changed, 9 insertions(+)

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diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 14d14beb1f7f..d27701199a4d 100644
--- a/include/linux/page_ref.h
+++ b/include/linux/page_ref.h
@@ -102,6 +102,15 @@  static inline void page_ref_sub(struct page *page, int nr)
 		__page_ref_mod(page, -nr);
+static inline int page_ref_sub_return(struct page *page, int nr)
+	int ret = atomic_sub_return(nr, &page->_refcount);
+	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod_and_return))
+		__page_ref_mod_and_return(page, -nr, ret);
+	return ret;
 static inline void page_ref_inc(struct page *page)