mm: fix bad rss-counter if remap_file_pages raced migration

Fix some "Bad rss-counter state" reports on exit, arising from the
interaction between page migration and remap_file_pages(): zap_pte()
must count a migration entry when zapping it.

And yes, it is possible (though very unusual) to find an anon page or
swap entry in a VM_SHARED nonlinear mapping: coming from that horrid
get_user_pages(write, force) case which COWs even in a shared mapping.

Signed-off-by: Hugh Dickins <hughd@google.com>
Tested-by: Sasha Levin sasha.levin@oracle.com>
Tested-by: Dave Jones davej@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 6 deletions

diff --git a/mm/fremap.c b/mm/fremap.c
index bbc4d660221a..34feba60a17e 100644
--- a/mm/fremap.c
+++ b/mm/fremap.c
@@ -23,28 +23,44 @@
 
 #include "internal.h"
 
+static int mm_counter(struct page *page)
+{
+        return PageAnon(page) ? MM_ANONPAGES : MM_FILEPAGES;
+}
+
 static void zap_pte(struct mm_struct *mm, struct vm_area_struct *vma,
                         unsigned long addr, pte_t *ptep)
 {
         pte_t pte = *ptep;
+        struct page *page;
+        swp_entry_t entry;
 
         if (pte_present(pte)) {
-                struct page *page;
-
                 flush_cache_page(vma, addr, pte_pfn(pte));
                 pte = ptep_clear_flush(vma, addr, ptep);
                 page = vm_normal_page(vma, addr, pte);
                 if (page) {
                         if (pte_dirty(pte))
                                 set_page_dirty(page);
+                        update_hiwater_rss(mm);
+                        dec_mm_counter(mm, mm_counter(page));
                         page_remove_rmap(page);
                         page_cache_release(page);
+                }
+        } else {        /* zap_pte() is not called when pte_none() */
+                if (!pte_file(pte)) {
                         update_hiwater_rss(mm);
-                        dec_mm_counter(mm, MM_FILEPAGES);
+                        entry = pte_to_swp_entry(pte);
+                        if (non_swap_entry(entry)) {
+                                if (is_migration_entry(entry)) {
+                                        page = migration_entry_to_page(entry);
+                                        dec_mm_counter(mm, mm_counter(page));
+                                }
+                        } else {
+                                free_swap_and_cache(entry);
+                                dec_mm_counter(mm, MM_SWAPENTS);
+                        }
                 }
-        } else {
-                if (!pte_file(pte))
-                        free_swap_and_cache(pte_to_swp_entry(pte));
                 pte_clear_not_present_full(mm, addr, ptep, 0);
         }
 }