From: Linus Torvalds torvalds@linux-foundation.org
commit 88b1a17dfc3ed7728316478fae0f5ad508f50397 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- include/linux/mm.h | 9 +++++++++ 1 file changed, 9 insertions(+)
--- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -930,6 +930,15 @@ static inline void get_page(struct page page_ref_inc(page); }
+static inline __must_check bool try_get_page(struct page *page) +{ + page = compound_head(page); + if (WARN_ON_ONCE(page_ref_count(page) <= 0)) + return false; + page_ref_inc(page); + return true; +} + static inline void put_page(struct page *page) { page = compound_head(page);