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From:   John Hubbard <jhubbard@nvidia.com>
To:     Andrew Morton <akpm@linux-foundation.org>
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Subject: [PATCH v4 04/12] mm: introduce page_ref_sub_return()
Date:   Tue, 4 Feb 2020 15:41:09 -0800
Message-ID: <20200204234117.2974687-5-jhubbard@nvidia.com>
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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.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/page_ref.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
index 14d14beb1f7f..a0e171265b79 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);
 }
=20
+static inline int page_ref_sub_return(struct page *page, int nr)
+{
+	int ret =3D atomic_sub_return(nr, &page->_refcount);
+
+	if (page_ref_tracepoint_active(__tracepoint_page_ref_mod))
+		__page_ref_mod_and_return(page, -nr, ret);
+	return ret;
+}
+
 static inline void page_ref_inc(struct page *page)
 {
 	atomic_inc(&page->_refcount);
--=20
2.25.0