[RFC PATCH] mm: Add helpers for locked_vm

[Alexey Kardashevskiy <aik@ozlabs.ru>]

This adds 2 helpers to change the locked_vm counter:
- try_increase_locked_vm - may fail if new locked_vm value will be greater
than the RLIMIT_MEMLOCK limit;
- decrease_locked_vm.

These will be used by drivers capable of locking memory by userspace
request. For example, VFIO can use it to check if it can lock DMA memory
or PPC-KVM can use it to check if it can lock memory for TCE tables.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
 include/linux/mm.h |  3 +++
 mm/mlock.c         | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e03dd29..1cb219d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2113,5 +2113,8 @@ void __init setup_nr_node_ids(void);
 static inline void setup_nr_node_ids(void) {}
 #endif
 
+extern long try_increment_locked_vm(long npages);
+extern void decrement_locked_vm(long npages);
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/mm/mlock.c b/mm/mlock.c
index b1eb536..39e4b55 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -864,3 +864,52 @@ void user_shm_unlock(size_t size, struct user_struct *user)
 	spin_unlock(&shmlock_user_lock);
 	free_uid(user);
 }
+
+/**
+ * try_increment_locked_vm() - checks if new locked_vm value is going to
+ * be less than RLIMIT_MEMLOCK and increments it by npages if it is.
+ *
+ * @npages: the number of pages to add to locked_vm.
+ *
+ * Returns 0 if succeeded or negative value if failed.
+ */
+long try_increment_locked_vm(long npages)
+{
+	long ret = 0, locked, lock_limit;
+
+	if (!current || !current->mm)
+		return -ESRCH; /* process exited */
+
+	down_write(&current->mm->mmap_sem);
+	locked = current->mm->locked_vm + npages;
+	lock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+	if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
+		pr_warn("RLIMIT_MEMLOCK (%ld) exceeded\n",
+				rlimit(RLIMIT_MEMLOCK));
+		ret = -ENOMEM;
+	} else {
+		current->mm->locked_vm += npages;
+	}
+	up_write(&current->mm->mmap_sem);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(try_increment_locked_vm);
+
+/**
+ * decrement_locked_vm() - decrements the current task's locked_vm counter.
+ *
+ * @npages: the number to decrement by.
+ */
+void decrement_locked_vm(long npages)
+{
+	if (!current || !current->mm)
+		return; /* process exited */
+
+	down_write(&current->mm->mmap_sem);
+	if (npages > current->mm->locked_vm)
+		npages = current->mm->locked_vm;
+	current->mm->locked_vm -= npages;
+	up_write(&current->mm->mmap_sem);
+}
+EXPORT_SYMBOL_GPL(decrement_locked_vm);
-- 
2.0.0

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