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Subject[RFC 5/8] mm: avoid using set_page_count() when pages are freed into allocator
Date
When struct pages are first initialized the page->_refcount field is
set 1. However, later when pages are freed into allocator we set
_refcount to 0 via set_page_count(). Unconditionally resetting
_refcount is dangerous.

Instead use page_ref_dec_return(), and verify that the _refcount is
what is expected.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
mm/page_alloc.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6af4596bddc2..9d18e5f9a85a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1659,6 +1659,7 @@ void __free_pages_core(struct page *page, unsigned int order)
unsigned int nr_pages = 1 << order;
struct page *p = page;
unsigned int loop;
+ int refcnt;

/*
* When initializing the memmap, __init_single_page() sets the refcount
@@ -1669,10 +1670,12 @@ void __free_pages_core(struct page *page, unsigned int order)
for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
prefetchw(p + 1);
__ClearPageReserved(p);
- set_page_count(p, 0);
+ refcnt = page_ref_dec_return(p);
+ VM_BUG_ON_PAGE(refcnt, p);
}
__ClearPageReserved(p);
- set_page_count(p, 0);
+ refcnt = page_ref_dec_return(p);
+ VM_BUG_ON_PAGE(refcnt, p);

atomic_long_add(nr_pages, &page_zone(page)->managed_pages);

@@ -2244,10 +2247,12 @@ void __init init_cma_reserved_pageblock(struct page *page)
{
unsigned i = pageblock_nr_pages;
struct page *p = page;
+ int refcnt;

do {
__ClearPageReserved(p);
- set_page_count(p, 0);
+ refcnt = page_ref_dec_return(p);
+ VM_BUG_ON_PAGE(refcnt, p);
} while (++p, --i);

set_pageblock_migratetype(page, MIGRATE_CMA);
--
2.33.0.1079.g6e70778dc9-goog
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