On Fri, 2015-01-30 at 19:08 +0700, Arseny Solokha wrote:

MMU_NO_CONTEXT is conditionally defined as 0 or (unsigned int)-1.

For nohash it is specifically -1.

 However, in __flush_tlb_page() a corresponding variable is only tested
for open coded 0, which can cause NULL pointer dereference if `mm'
argument was legitimately passed as such.

Bail out early in case the first argument is NULL, thus eliminate confusion
between different values of MMU_NO_CONTEXT and avoid disabling and then
re-enabling preemption unnecessarily.

How did you notice this?  Did you see an oops, or was it code
inspection?  I'm wondering what codepath gets here with mm == NULL.

-Scott