Commit f958d7b5 authored Apr 11, 2019 by Linus Torvalds

mm: make page ref count overflow check tighter and more explicit



We have a VM_BUG_ON() to check that the page reference count doesn't
underflow (or get close to overflow) by checking the sign of the count.

That's all fine, but we actually want to allow people to use a "get page
ref unless it's already very high" helper function, and we want that one
to use the sign of the page ref (without triggering this VM_BUG_ON).

Change the VM_BUG_ON to only check for small underflows (or _very_ close
to overflowing), and ignore overflows which have strayed into negative
territory.

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>

parent 1c163f4c

include/linux/mm.h

+5 −1

Original line number	Diff line number	Diff line
		@@ -965,6 +965,10 @@ static inline bool is_pci_p2pdma_page(const struct page *page)
		}
		#endif /* CONFIG_DEV_PAGEMAP_OPS */

		/* 127: arbitrary random number, small enough to assemble well */
		#define page_ref_zero_or_close_to_overflow(page) \
		((unsigned int) page_ref_count(page) + 127u <= 127u)

		static inline void get_page(struct page *page)
		{
		page = compound_head(page);
		@@ -972,7 +976,7 @@ static inline void get_page(struct page *page)
		* Getting a normal page or the head of a compound page
		* requires to already have an elevated page->_refcount.
		*/
		VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
		VM_BUG_ON_PAGE(page_ref_zero_or_close_to_overflow(page), page);
		page_ref_inc(page);
		}

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