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authorLinus Torvalds <torvalds@linux-foundation.org>2019-04-11 10:06:20 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-04-14 10:00:04 -0700
commitf958d7b528b1b40c44cfda5eabe2d82760d868c3 (patch)
tree4064635cb1fdb10668227084cb68c2c4be2fb0f8
parent1c163f4c7b3f621efff9b28a47abb36f7378d783 (diff)
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>
-rw-r--r--include/linux/mm.h6
1 files changed, 5 insertions, 1 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80bb6408fe73..541d99b86aea 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -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);
}