[040/155] mm: introduce page_ref_sub_return()

Commit Message

Andrew Morton April 2, 2020, 4:05 a.m. UTC

From: John Hubbard <[e-mail]>
Subject: mm: introduce page_ref_sub_return()

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.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: John Hubbard <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Cc: Ira Weiny <[e-mail]>
Cc: Jérôme Glisse <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/page_ref.h |    9 +++++++++
 1 file changed, 9 insertions(+)

Patch

--- a/include/linux/page_ref.h~mm-introduce-page_ref_sub_return
+++ a/include/linux/page_ref.h
@@ -102,6 +102,15 @@  static inline void page_ref_sub(struct p
 		__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)
 {
 	atomic_inc(&page->_refcount);