mm/mlock.c: document scary-looking stack expansion mlock chain

The fact that mlock calls get_user_pages, and get_user_pages might call
mlock when expanding a stack looks like a potential recursion.

However, mlock makes sure the requested range is already contained
within a vma, so no stack expansion will actually happen from mlock.

Should this ever change: the stack expansion mlocks only the newly
expanded range and so will not result in recursive expansion.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mlock.c b/mm/mlock.c
index d6378fe..38db3b0 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -185,6 +185,10 @@ long __mlock_vma_pages_range(struct vm_area_struct *vma,
        if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
                gup_flags |= FOLL_FORCE;
 
+       /*
+        * We made sure addr is within a VMA, so the following will
+        * not result in a stack expansion that recurses back here.
+        */
        return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
                                NULL, NULL, nonblocking);
 }