On Tue, Oct 26, 2021 at 2:45 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
On 10/26/21 10:38 AM, Pasha Tatashin wrote:
prep_compound_gigantic_page() calls set_page_count(0, p), but it is not
needed because page_ref_freeze(p, 1) already sets refcount to 0.
Using, set_page_count() is dangerous, because it unconditionally resets
refcount from the current value to unrestrained value, and therefore
should be minimized.
Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
Thanks!
My bad for not removing the set_page_count when adding the page_ref_freeze.
FYI, there have been additional changes to this routine in Andrew's
tree. Not really sure if we want/need the VM_BUG_ON_PAGE as that would
only check if there was a 'bug' in page_ref_freeze.
I would like to keep it. Part of the idea of this series is to reduce
reliance on comments such as:
/* No worries, refcount is A therefore we can do B */
And instead enforce that via VM_BUG_ON(). It should be able to
prevent existing and future _refcount related bugs from manifesting as
memory corruptions.
Pasha
--
Mike Kravetz
---
mm/hugetlb.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 95dc7b83381f..7e3996c8b696 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1707,7 +1707,7 @@ static bool prep_compound_gigantic_page(struct page *page, unsigned int order)
pr_warn("HugeTLB page can not be used due to unexpected inflated ref count\n");
goto out_error;
}
- set_page_count(p, 0);
+ VM_BUG_ON_PAGE(page_count(p), p);
set_compound_head(p, page);
}
atomic_set(compound_mapcount_ptr(page), -1);
