Re: [PATCH 5/7] mm: free user PTE page table pages

From: Qi Zheng
Date: Mon Jul 19 2021 - 08:56:10 EST

On 7/18/21 2:19 PM, Mika Penttilä wrote:

+ * returns true if the pmd has been populated with PTE page table,
+ * or false for all other cases.
+ */
+bool pte_install_try_get(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte)
+    spinlock_t *ptl;
+    bool retval = true;
+    ptl = pmd_lock(mm, pmd);
+    if (likely(pmd_none(*pmd))) {
+        __pte_install(mm, pmd, pte);
+    } else if (pmd_leaf(*pmd) || !pmd_present(*pmd)) {
+        retval = false;
+    } else if (!pte_get_unless_zero(pmd)) {
+        spin_unlock(ptl);
+        goto retry;
+    }
+    spin_unlock(ptl);
+    return retval;

Can pte_get_unless_zero() return true above? Can the pmd have been by populated by others? In that case the ref count is wrongly incremented.

Here we only have mmap_read_lock(mm), so the pmd can be populated with
other PTE page table page after a page fault in a different thread B of this mm. In this case, thread B already hold a pte_refcount of the PTE page table page populated in the pmd, so pte_get_unless_zero() can
return true above.

Similarly, if THP is enabled, the pmd also can be populated with a THP page, we can see more detail in comment in handle_pte_fault(). The
pmd_leaf() above is to detect this situation.