mm: m(un)lock avoid ZERO_PAGE

I'm still reluctant to clutter __get_user_pages() with another flag, just
to avoid touching ZERO_PAGE count in mlock(); though we can add that later
if it shows up as an issue in practice.

But when mlocking, we can test page->mapping slightly earlier, to avoid
the potentially bouncy rescheduling of lock_page on ZERO_PAGE - mlock
didn't lock_page in olden ZERO_PAGE days, so we might have regressed.

And when munlocking, it turns out that FOLL_DUMP coincidentally does
what's needed to avoid all updates to ZERO_PAGE, so use that here also.
Plus add comment suggested by KAMEZAWA Hiroyuki.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 36 insertions, 13 deletions

diff --git a/mm/mlock.c b/mm/mlock.c
index 22041aa9f5c1..bd6f0e466f6c 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -198,17 +198,26 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
                 for (i = 0; i < ret; i++) {
                         struct page *page = pages[i];
 
-                        lock_page(page);
-                        /*
-                         * Because we lock page here and migration is blocked
-                         * by the elevated reference, we need only check for
-                         * file-cache page truncation.  This page->mapping
-                         * check also neatly skips over the ZERO_PAGE(),
-                         * though if that's common we'd prefer not to lock it.
-                         */
-                        if (page->mapping)
-                                mlock_vma_page(page);
-                        unlock_page(page);
+                        if (page->mapping) {
+                                /*
+                                 * That preliminary check is mainly to avoid
+                                 * the pointless overhead of lock_page on the
+                                 * ZERO_PAGE: which might bounce very badly if
+                                 * there is contention.  However, we're still
+                                 * dirtying its cacheline with get/put_page:
+                                 * we'll add another __get_user_pages flag to
+                                 * avoid it if that case turns out to matter.
+                                 */
+                                lock_page(page);
+                                /*
+                                 * Because we lock page here and migration is
+                                 * blocked by the elevated reference, we need
+                                 * only check for file-cache page truncation.
+                                 */
+                                if (page->mapping)
+                                        mlock_vma_page(page);
+                                unlock_page(page);
+                        }
                         put_page(page); /* ref from get_user_pages() */
                 }
 
@@ -309,9 +318,23 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
         vma->vm_flags &= ~VM_LOCKED;
 
         for (addr = start; addr < end; addr += PAGE_SIZE) {
-                struct page *page = follow_page(vma, addr, FOLL_GET);
-                if (page) {
+                struct page *page;
+                /*
+                 * Although FOLL_DUMP is intended for get_dump_page(),
+                 * it just so happens that its special treatment of the
+                 * ZERO_PAGE (returning an error instead of doing get_page)
+                 * suits munlock very well (and if somehow an abnormal page
+                 * has sneaked into the range, we won't oops here: great).
+                 */
+                page = follow_page(vma, addr, FOLL_GET | FOLL_DUMP);
+                if (page && !IS_ERR(page)) {
                         lock_page(page);
+                        /*
+                         * Like in __mlock_vma_pages_range(),
+                         * because we lock page here and migration is
+                         * blocked by the elevated reference, we need
+                         * only check for file-cache page truncation.
+                         */
                         if (page->mapping)
                                 munlock_vma_page(page);
                         unlock_page(page);