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Subject[PATCH 03/10] mm: Avoid using set_page_count() in set_page_recounted()
Date
set_page_refcounted() converts a non-refcounted page that has
(page->_refcount == 0) into a refcounted page by setting _refcount to
1.

The current apporach uses the following logic:

VM_BUG_ON_PAGE(page_ref_count(page), page);
set_page_count(page, 1);

However, if _refcount changes from 0 to 1 between the VM_BUG_ON_PAGE()
and set_page_count() we can break _refcount, which can cause other
problems such as memory corruptions.

Instead, use a safer method: increment _refcount first and verify
that at increment time it was indeed 1.

refcnt = page_ref_inc_return(page);
VM_BUG_ON_PAGE(refcnt != 1, page);

Use page_ref_inc_return() to avoid unconditionally overwriting
the _refcount value with set_page_count(), and check the return value.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
mm/internal.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 94909fcce671..47d1d3c892fb 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -132,9 +132,11 @@ static inline bool page_evictable(struct page *page)
*/
static inline void set_page_refcounted(struct page *page)
{
+ int refcnt;
+
VM_BUG_ON_PAGE(PageTail(page), page);
- VM_BUG_ON_PAGE(page_ref_count(page), page);
- set_page_count(page, 1);
+ refcnt = page_ref_inc_return(page);
+ VM_BUG_ON_PAGE(refcnt != 1, page);
}

extern unsigned long highest_memmap_pfn;
--
2.34.1.400.ga245620fadb-goog
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