[PATCH] mm: follow_page with inner ptlock

Final step in pushing down common core's page_table_lock.  follow_page no
longer wants caller to hold page_table_lock, uses pte_offset_map_lock itself;
and so no page_table_lock is taken in get_user_pages itself.

But get_user_pages (and get_futex_key) do then need follow_page to pin the
page for them: take Daniel's suggestion of bitflags to follow_page.

Need one for WRITE, another for TOUCH (it was the accessed flag before:
vanished along with check_user_page_readable, but surely get_numa_maps is
wrong to mark every page it finds as accessed), another for GET.

And another, ANON to dispose of untouched_anonymous_page: it seems silly for
that to descend a second time, let follow_page observe if there was no page
table and return ZERO_PAGE if so.  Fix minor bug in that: check VM_LOCKED -
make_pages_present ought to make readonly anonymous present.

Give get_numa_maps a cond_resched while we're there.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index ca05fe6a70b2..3b4d5ad44cc6 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -205,15 +205,13 @@ static int get_futex_key(unsigned long uaddr, union futex_key *key)
         /*
          * Do a quick atomic lookup first - this is the fastpath.
          */
-        spin_lock(&current->mm->page_table_lock);
-        page = follow_page(mm, uaddr, 0);
+        page = follow_page(mm, uaddr, FOLL_TOUCH|FOLL_GET);
         if (likely(page != NULL)) {
                 key->shared.pgoff =
                         page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
-                spin_unlock(&current->mm->page_table_lock);
+                put_page(page);
                 return 0;
         }
-        spin_unlock(&current->mm->page_table_lock);
 
         /*
          * Do it the general way.