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Subject: [PATCH 2/8] mm: add 'try_get_page()' helper function
Date:   Tue, 23 Jul 2019 16:38:25 +0530
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From: Linus Torvalds <torvalds@linux-foundation.org>

commit 88b1a17dfc3ed7728316478fae0f5ad508f50397 upsteam.

This is the same as the traditional 'get_page()' function, but instead
of unconditionally incrementing the reference count of the page, it only
does so if the count was "safe".  It returns whether the reference count
was incremented (and is marked __must_check, since the caller obviously
has to be aware of it).

Also like 'get_page()', you can't use this function unless you already
had a reference to the page.  The intent is that you can use this
exactly like get_page(), but in situations where you want to limit the
maximum reference count.

The code currently does an unconditional WARN_ON_ONCE() if we ever hit
the reference count issues (either zero or negative), as a notification
that the conditional non-increment actually happened.

NOTE! The count access for the "safety" check is inherently racy, but
that doesn't matter since the buffer we use is basically half the range
of the reference count (ie we look at the sign of the count).

Acked-by: Matthew Wilcox <willy@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ 4.4.y backport notes:
  Srivatsa:
  - Adapted try_get_page() to match the get_page()
    implementation in 4.4.y, except for the refcount check.
  - Added try_get_page_foll() which will be needed
    in a subsequent patch. ]
Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Ajay Kaher <akaher@vmware.com>
---
 include/linux/mm.h | 12 ++++++++++++
 mm/internal.h      | 23 +++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 701088e..52edaf1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -505,6 +505,18 @@ static inline void get_page(struct page *page)
 	atomic_inc(&page->_count);
 }
 
+static inline __must_check bool try_get_page(struct page *page)
+{
+	if (unlikely(PageTail(page)))
+		if (likely(__get_page_tail(page)))
+			return true;
+
+	if (WARN_ON_ONCE(atomic_read(&page->_count) <= 0))
+		return false;
+	atomic_inc(&page->_count);
+	return true;
+}
+
 static inline struct page *virt_to_head_page(const void *x)
 {
 	struct page *page = virt_to_page(x);
diff --git a/mm/internal.h b/mm/internal.h
index 67015e5..d83afc9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -112,6 +112,29 @@ static inline void get_page_foll(struct page *page)
 	}
 }
 
+static inline __must_check bool try_get_page_foll(struct page *page)
+{
+	if (unlikely(PageTail(page))) {
+		if (WARN_ON_ONCE(atomic_read(&compound_head(page)->_count) <= 0))
+			return false;
+		/*
+		 * This is safe only because
+		 * __split_huge_page_refcount() can't run under
+		 * get_page_foll() because we hold the proper PT lock.
+		 */
+		__get_page_tail_foll(page, true);
+	} else {
+		/*
+		 * Getting a normal page or the head of a compound page
+		 * requires to already have an elevated page->_count.
+		 */
+		if (WARN_ON_ONCE(atomic_read(&page->_count) <= 0))
+			return false;
+		atomic_inc(&page->_count);
+	}
+	return true;
+}
+
 extern unsigned long highest_memmap_pfn;
 
 /*
-- 
2.7.4