[PATCH] numa: Change get_mempolicy() to use nr_node_ids instead of MAX_NUMNODES

[<rcampbell@nvidia.com>]

From: Ralph Campbell <rcampbell@nvidia.com>

The system call, get_mempolicy() [1], passes an unsigned long *nodemask
pointer and an unsigned long maxnode argument which specifies the
length of the user's nodemask array in bits (which is rounded up).
The manual page says that if the maxnode value is too small,
get_mempolicy will return EINVAL but there is no system call to return
this minimum value. To determine this value, some programs search
/proc/<pid>/status for a line starting with "Mems_allowed:" and use
the number of digits in the mask to determine the minimum value.
A recent change to the way this line is formatted [2] causes these
programs to compute a value less than MAX_NUMNODES so get_mempolicy()
returns EINVAL.

Change get_mempolicy(), the older compat version of get_mempolicy(), and
the copy_nodes_to_user() function to use nr_node_ids instead of
MAX_NUMNODES, thus preserving the defacto method of computing the
minimum size for the nodemask array and the maxnode argument.

[1] http://man7.org/linux/man-pages/man2/get_mempolicy.2.html
[2] https://lore.kernel.org/lkml/1545405631-6808-1-git-send-email-longman@redhat.com

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 1da2f1f09675..af171ccb56a2 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1314,7 +1314,7 @@ static int copy_nodes_to_user(unsigned long __user *mask, unsigned long maxnode,
 			      nodemask_t *nodes)
 {
 	unsigned long copy = ALIGN(maxnode-1, 64) / 8;
-	const int nbytes = BITS_TO_LONGS(MAX_NUMNODES) * sizeof(long);
+	unsigned int nbytes = BITS_TO_LONGS(nr_node_ids) * sizeof(long);
 
 	if (copy > nbytes) {
 		if (copy > PAGE_SIZE)
@@ -1491,7 +1491,7 @@ static int kernel_get_mempolicy(int __user *policy,
 	int uninitialized_var(pval);
 	nodemask_t nodes;
 
-	if (nmask != NULL && maxnode < MAX_NUMNODES)
+	if (nmask != NULL && maxnode < nr_node_ids)
 		return -EINVAL;
 
 	err = do_get_mempolicy(&pval, &nodes, addr, flags);
@@ -1527,7 +1527,7 @@ COMPAT_SYSCALL_DEFINE5(get_mempolicy, int __user *, policy,
 	unsigned long nr_bits, alloc_size;
 	DECLARE_BITMAP(bm, MAX_NUMNODES);
 
-	nr_bits = min_t(unsigned long, maxnode-1, MAX_NUMNODES);
+	nr_bits = min_t(unsigned long, maxnode-1, nr_node_ids);
 	alloc_size = ALIGN(nr_bits, BITS_PER_LONG) / 8;
 
 	if (nmask)
-- 
2.17.2