mm: pass mm to grab_swap_token

If a kthread happens to use get_user_pages() on an mm (as KSM does),
there's a chance that it will end up trying to read in a swap page, then
oops in grab_swap_token() because the kthread has no mm: GUP passes down
the right mm, so grab_swap_token() ought to be using it.

We have not identified a stronger case than KSM's daemon (not yet in
mainline), but the issue must have come up before, since RHEL has included
a fix for this for years (though a different fix, they just back out of
grab_swap_token if current->mm is unset: which is what we first proposed,
but using the right mm here seems more correct).

Reported-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 17 deletions

diff --git a/mm/thrash.c b/mm/thrash.c
index c4c5205a9c3..2372d4ed5dd 100644
--- a/mm/thrash.c
+++ b/mm/thrash.c
@@ -26,47 +26,45 @@ static DEFINE_SPINLOCK(swap_token_lock);
 struct mm_struct *swap_token_mm;
 static unsigned int global_faults;
 
-void grab_swap_token(void)
+void grab_swap_token(struct mm_struct *mm)
 {
         int current_interval;
 
         global_faults++;
 
-        current_interval = global_faults - current->mm->faultstamp;
+        current_interval = global_faults - mm->faultstamp;
 
         if (!spin_trylock(&swap_token_lock))
                 return;
 
         /* First come first served */
         if (swap_token_mm == NULL) {
-                current->mm->token_priority = current->mm->token_priority + 2;
-                swap_token_mm = current->mm;
+                mm->token_priority = mm->token_priority + 2;
+                swap_token_mm = mm;
                 goto out;
         }
 
-        if (current->mm != swap_token_mm) {
-                if (current_interval < current->mm->last_interval)
-                        current->mm->token_priority++;
+        if (mm != swap_token_mm) {
+                if (current_interval < mm->last_interval)
+                        mm->token_priority++;
                 else {
-                        if (likely(current->mm->token_priority > 0))
-                                current->mm->token_priority--;
+                        if (likely(mm->token_priority > 0))
+                                mm->token_priority--;
                 }
                 /* Check if we deserve the token */
-                if (current->mm->token_priority >
-                                swap_token_mm->token_priority) {
-                        current->mm->token_priority += 2;
-                        swap_token_mm = current->mm;
+                if (mm->token_priority > swap_token_mm->token_priority) {
+                        mm->token_priority += 2;
+                        swap_token_mm = mm;
                 }
         } else {
                 /* Token holder came in again! */
-                current->mm->token_priority += 2;
+                mm->token_priority += 2;
         }
 
 out:
-        current->mm->faultstamp = global_faults;
-        current->mm->last_interval = current_interval;
+        mm->faultstamp = global_faults;
+        mm->last_interval = current_interval;
         spin_unlock(&swap_token_lock);
-return;
 }
 
 /* Called on process exit. */