1 files changed, 42 insertions, 13 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 29bc6e471df4..cb7be110cad3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2117,7 +2117,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 {
         struct page *page;
         pte_t *_pte;
-        int referenced = 0, none = 0;
+        int none = 0;
+        bool referenced = false, writable = false;
         for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
              _pte++, address += PAGE_SIZE) {
                 pte_t pteval = *_pte;
@@ -2127,7 +2128,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                         else
                                 goto out;
                 }
-                if (!pte_present(pteval) || !pte_write(pteval))
+                if (!pte_present(pteval))
                         goto out;
                 page = vm_normal_page(vma, address, pteval);
                 if (unlikely(!page))
@@ -2137,9 +2138,6 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                 VM_BUG_ON_PAGE(!PageAnon(page), page);
                 VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
 
-                /* cannot use mapcount: can't collapse if there's a gup pin */
-                if (page_count(page) != 1)
-                        goto out;
                 /*
                  * We can do it before isolate_lru_page because the
                  * page can't be freed from under us. NOTE: PG_lock
@@ -2148,6 +2146,29 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                  */
                 if (!trylock_page(page))
                         goto out;
+
+                /*
+                 * cannot use mapcount: can't collapse if there's a gup pin.
+                 * The page must only be referenced by the scanned process
+                 * and page swap cache.
+                 */
+                if (page_count(page) != 1 + !!PageSwapCache(page)) {
+                        unlock_page(page);
+                        goto out;
+                }
+                if (pte_write(pteval)) {
+                        writable = true;
+                } else {
+                        if (PageSwapCache(page) && !reuse_swap_page(page)) {
+                                unlock_page(page);
+                                goto out;
+                        }
+                        /*
+                         * Page is not in the swap cache. It can be collapsed
+                         * into a THP.
+                         */
+                }
+
                 /*
                  * Isolate the page to avoid collapsing an hugepage
                  * currently in use by the VM.
@@ -2164,9 +2185,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
                 /* If there is no mapped pte young don't collapse the page */
                 if (pte_young(pteval) || PageReferenced(page) ||
                     mmu_notifier_test_young(vma->vm_mm, address))
-                        referenced = 1;
+                        referenced = true;
         }
-        if (likely(referenced))
+        if (likely(referenced && writable))
                 return 1;
 out:
         release_pte_pages(pte, _pte);
@@ -2519,11 +2540,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 {
         pmd_t *pmd;
         pte_t *pte, *_pte;
-        int ret = 0, referenced = 0, none = 0;
+        int ret = 0, none = 0;
         struct page *page;
         unsigned long _address;
         spinlock_t *ptl;
         int node = NUMA_NO_NODE;
+        bool writable = false, referenced = false;
 
         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
@@ -2542,8 +2564,11 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
                         else
                                 goto out_unmap;
                 }
-                if (!pte_present(pteval) || !pte_write(pteval))
+                if (!pte_present(pteval))
                         goto out_unmap;
+                if (pte_write(pteval))
+                        writable = true;
+
                 page = vm_normal_page(vma, _address, pteval);
                 if (unlikely(!page))
                         goto out_unmap;
@@ -2560,14 +2585,18 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
                 VM_BUG_ON_PAGE(PageCompound(page), page);
                 if (!PageLRU(page) || PageLocked(page) || !PageAnon(page))
                         goto out_unmap;
-                /* cannot use mapcount: can't collapse if there's a gup pin */
-                if (page_count(page) != 1)
+                /*
+                 * cannot use mapcount: can't collapse if there's a gup pin.
+                 * The page must only be referenced by the scanned process
+                 * and page swap cache.
+                 */
+                if (page_count(page) != 1 + !!PageSwapCache(page))
                         goto out_unmap;
                 if (pte_young(pteval) || PageReferenced(page) ||
                     mmu_notifier_test_young(vma->vm_mm, address))
-                        referenced = 1;
+                        referenced = true;
         }
-        if (referenced)
+        if (referenced && writable)
                 ret = 1;
 out_unmap:
         pte_unmap_unlock(pte, ptl);