page_cache_get(old_page);
- /*
- * Only catch write-faults on shared writable pages,
- * read-only shared pages can get COWed by
- * get_user_pages(.write=1, .force=1).
- */
if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
int tmp;
if (unlikely(pmd_none(*pmd)) &&
unlikely(__pte_alloc(mm, vma, pmd, address)))
return VM_FAULT_OOM;
- /* if an huge pmd materialized from under us just retry later */
- if (unlikely(pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
+ /*
+ * If a huge pmd materialized under us just retry later. Use
+ * pmd_trans_unstable() instead of pmd_trans_huge() to ensure the pmd
+ * didn't become pmd_trans_huge under us and then back to pmd_none, as
+ * a result of MADV_DONTNEED running immediately after a huge pmd fault
+ * in a different thread of this mm, in turn leading to a misleading
+ * pmd_trans_huge() retval. All we have to ensure is that it is a
+ * regular pmd that we can walk with pte_offset_map() and we can do that
+ * through an atomic read in C, which is what pmd_trans_unstable()
+ * provides.
+ */
+ if (unlikely(pmd_trans_unstable(pmd) || pmd_devmap(*pmd)))
return 0;
/*
* A regular pmd is established and it can't morph into a huge pmd