mm, mempolicy: make mpol_to_str robust and always succeed

mpol_to_str() should not fail.  Currently, it either fails because the
string buffer is too small or because a string hasn't been defined for a
mempolicy mode.

If a new mempolicy mode is introduced and no string is defined for it,
just warn and return "unknown".

If the buffer is too small, just truncate the string and return, the
same behavior as snprintf().

This also fixes a bug where there was no NULL-byte termination when doing
*p++ = '=' and *p++ ':' and maxlen has been reached.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Chen Gang <gang.chen@asianux.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 8 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 390bdab01c3c..9f1369fe0afb 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1387,8 +1387,8 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
         struct mm_struct *mm = vma->vm_mm;
         struct mm_walk walk = {};
         struct mempolicy *pol;
-        int n;
-        char buffer[50];
+        char buffer[64];
+        int nid;
 
         if (!mm)
                 return 0;
@@ -1404,10 +1404,8 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
         walk.mm = mm;
 
         pol = get_vma_policy(task, vma, vma->vm_start);
-        n = mpol_to_str(buffer, sizeof(buffer), pol);
+        mpol_to_str(buffer, sizeof(buffer), pol);
         mpol_cond_put(pol);
-        if (n < 0)
-                return n;
 
         seq_printf(m, "%08lx %s", vma->vm_start, buffer);
 
@@ -1460,9 +1458,9 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
         if (md->writeback)
                 seq_printf(m, " writeback=%lu", md->writeback);
 
-        for_each_node_state(n, N_MEMORY)
-                if (md->node[n])
-                        seq_printf(m, " N%d=%lu", n, md->node[n]);
+        for_each_node_state(nid, N_MEMORY)
+                if (md->node[nid])
+                        seq_printf(m, " N%d=%lu", nid, md->node[nid]);
 out:
         seq_putc(m, '\n');