mm: simplify page migration's anon_vma comment and flow

[mirror_ubuntu-eoan-kernel.git] / mm / migrate.c

diff --git a/mm/migrate.c b/mm/migrate.c
index 7b44ebdf2d2684a1dac8ab413592d39c5e775e92..08a7b6c4c266343e0afe67e553e46c1916902c23 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -819,6 +819,7 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
                        goto out_unlock;
                wait_on_page_writeback(page);
        }
+
        /*
         * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
         * we cannot notice that anon_vma is freed while we migrates a page.
@@ -826,34 +827,15 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
         * of migration. File cache pages are no problem because of page_lock()
         * File Caches may use write_page() or lock_page() in migration, then,
         * just care Anon page here.
+        *
+        * Only page_get_anon_vma() understands the subtleties of
+        * getting a hold on an anon_vma from outside one of its mms.
+        * But if we cannot get anon_vma, then we won't need it anyway,
+        * because that implies that the anon page is no longer mapped
+        * (and cannot be remapped so long as we hold the page lock).
         */
-       if (PageAnon(page) && !PageKsm(page)) {
-               /*
-                * Only page_lock_anon_vma_read() understands the subtleties of
-                * getting a hold on an anon_vma from outside one of its mms.
-                */
+       if (PageAnon(page) && !PageKsm(page))
                anon_vma = page_get_anon_vma(page);
-               if (anon_vma) {
-                       /*
-                        * Anon page
-                        */
-               } else if (PageSwapCache(page)) {
-                       /*
-                        * We cannot be sure that the anon_vma of an unmapped
-                        * swapcache page is safe to use because we don't
-                        * know in advance if the VMA that this page belonged
-                        * to still exists. If the VMA and others sharing the
-                        * data have been freed, then the anon_vma could
-                        * already be invalid.
-                        *
-                        * To avoid this possibility, swapcache pages get
-                        * migrated but are not remapped when migration
-                        * completes
-                        */
-               } else {
-                       goto out_unlock;
-               }
-       }
 
        /*
         * Block others from accessing the new page when we get around to
@@ -898,6 +880,8 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
                }
        } else if (page_mapped(page)) {
                /* Establish migration ptes */
+               VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && !anon_vma,
+                               page);
                try_to_unmap(page,
                        TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
                page_was_mapped = 1;