fork: reset mm->pinned_vm

mm->pinned_vm counts pages of mm's address space that were permanently
pinned in memory by increasing their reference counter. The counter was
introduced by commit bc3e53f ("mm: distinguish between mlocked and
pinned pages"), while before it locked_vm had been used for such pages.

Obviously, we should reset the counter on fork if !CLONE_VM, just like
we do with locked_vm, but currently we don't. Let's fix it.

This patch will fix the contents of /proc/pid/status:VmPin.

ib_umem_get[infiniband] and perf_mmap still check pinned_vm against
RLIMIT_MEMLOCK.  It's left from the times when pinned pages were accounted
under locked_vm, but today it looks wrong.  It isn't clear how we should
deal with it.

We still have some drivers accounting pinned pages under mm->locked_vm -
this is what commit bc3e53f was fighting against.  It's
infiniband/usnic and vfio.

Signed-off-by: Vladimir Davydov <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Roland Dreier <[email protected]>
Cc: Sean Hefty <[email protected]>
Cc: Hal Rosenstock <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/fork.c

@@ -543,6 +543,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
 	atomic_long_set(&mm->nr_ptes, 0);
 	mm->map_count = 0;
 	mm->locked_vm = 0;
+	mm->pinned_vm = 0;
 	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
 	spin_lock_init(&mm->page_table_lock);
 	mm_init_cpumask(mm);