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 Signed-off-by: Vlastimil Babka vbabka@suse.cz Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/mm.h | 12 ++++++++++++ mm/internal.h | 23 +++++++++++++++++++++++ 2 files changed, 35 insertions(+)
--- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -505,6 +505,18 @@ static inline void get_page(struct 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); --- a/mm/internal.h +++ b/mm/internal.h @@ -112,6 +112,29 @@ static inline void get_page_foll(struct } }
+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;
/*