mm: add new mmgrab() helper

Apart from adding the helper function itself, the rest of the kernel is
converted mechanically using:

  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_count);/mmgrab\(\1\);/'
  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_count);/mmgrab\(\&\1\);/'

This is needed for a later patch that hooks into the helper, but might
be a worthwhile cleanup on its own.

(Michal Hocko provided most of the kerneldoc comment.)

Link: http://lkml.kernel.org/r/20161218123229.22952-1-vegard.nossum@oracle.com
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index e6e0a619cb3a..3c421d06a18e 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1847,7 +1847,7 @@ static struct file *userfaultfd_file_create(int flags)
         ctx->released = false;
         ctx->mm = current->mm;
         /* prevent the mm struct to be freed */
-        atomic_inc(&ctx->mm->mm_count);
+        mmgrab(ctx->mm);
 
         file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
                                   O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));