mm/rmap: fix munlocking Anon THP with mlocked ptes

Many thanks to Kirill for reminding that PageDoubleMap cannot be relied on
to warn of pte mappings in the Anon THP case; and a scan of subpages does
not seem appropriate here.  Note how follow_trans_huge_pmd() does not even
mark an Anon THP as mlocked when compound_mapcount != 1: multiple mlocking
of Anon THP is avoided, so simply return from page_mlock() in this case.

Link: https://lore.kernel.org/lkml/cfa154c-d595-406-eb7d-eb9df730f944@google.com/
Fixes: d8d57d4013af ("mm/rmap: fix old bug: munlocking THP missed other mlocks")
Reported-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 795f9d5f8386360be2eed9d7a31345aa7803f04b..b9eb5c12f3fe10bc83ce0d26d3f804d7f252f517 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1440,21 +1440,20 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                /*
                 * If the page is mlock()d, we cannot swap it out.
                 */
-               if (!(flags & TTU_IGNORE_MLOCK)) {
-                       if (vma->vm_flags & VM_LOCKED) {
-                               /* PTE-mapped THP are never marked as mlocked */
-                               if (!PageTransCompound(page) ||
-                                   (PageHead(page) && !PageDoubleMap(page))) {
-                                       /*
-                                        * Holding pte lock, we do *not* need
-                                        * mmap_lock here
-                                        */
-                                       mlock_vma_page(page);
-                               }
-                               ret = false;
-                               page_vma_mapped_walk_done(&pvmw);
-                               break;
-                       }
+               if (!(flags & TTU_IGNORE_MLOCK) &&
+                   (vma->vm_flags & VM_LOCKED)) {
+                       /*
+                        * PTE-mapped THP are never marked as mlocked: so do
+                        * not set it on a DoubleMap THP, nor on an Anon THP
+                        * (which may still be PTE-mapped after DoubleMap was
+                        * cleared).  But stop unmapping even in those cases.
+                        */
+                       if (!PageTransCompound(page) || (PageHead(page) &&
+                            !PageDoubleMap(page) && !PageAnon(page)))
+                               mlock_vma_page(page);
+                       page_vma_mapped_walk_done(&pvmw);
+                       ret = false;
+                       break;
                }
 
                /* Unexpected PMD-mapped THP? */
@@ -1986,8 +1985,10 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma,
                 */
                if (vma->vm_flags & VM_LOCKED) {
                        /*
-                        * PTE-mapped THP are never marked as mlocked, but
-                        * this function is never called when PageDoubleMap().
+                        * PTE-mapped THP are never marked as mlocked; but
+                        * this function is never called on a DoubleMap THP,
+                        * nor on an Anon THP (which may still be PTE-mapped
+                        * after DoubleMap was cleared).
                         */
                        mlock_vma_page(page);
                        /*
@@ -2022,6 +2023,10 @@ void page_mlock(struct page *page)
        VM_BUG_ON_PAGE(!PageLocked(page) || PageLRU(page), page);
        VM_BUG_ON_PAGE(PageCompound(page) && PageDoubleMap(page), page);
 
+       /* Anon THP are only marked as mlocked when singly mapped */
+       if (PageTransCompound(page) && PageAnon(page))
+               return;
+
        rmap_walk(page, &rwc);
 }