optimize compat_core_sys_select() by a using stack space for small fd sets

Optimize select by a using stack space for small fd sets.
core_sys_select() already has this optimization.  This is for compat
version.

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 6 deletions

diff --git a/fs/compat.c b/fs/compat.c
index 1de2331db844..4db6216e5266 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -1544,9 +1544,10 @@ int compat_core_sys_select(int n, compat_ulong_t __user *inp,
         compat_ulong_t __user *outp, compat_ulong_t __user *exp, s64 *timeout)
 {
         fd_set_bits fds;
-        char *bits;
+        void *bits;
         int size, max_fds, ret = -EINVAL;
         struct fdtable *fdt;
+        long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
 
         if (n < 0)
                 goto out_nofds;
@@ -1564,11 +1565,14 @@ int compat_core_sys_select(int n, compat_ulong_t __user *inp,
          * since we used fdset we need to allocate memory in units of
          * long-words.
          */
-        ret = -ENOMEM;
         size = FDS_BYTES(n);
-        bits = kmalloc(6 * size, GFP_KERNEL);
-        if (!bits)
-                goto out_nofds;
+        bits = stack_fds;
+        if (size > sizeof(stack_fds) / 6) {
+                bits = kmalloc(6 * size, GFP_KERNEL);
+                ret = -ENOMEM;
+                if (!bits)
+                        goto out_nofds;
+        }
         fds.in      = (unsigned long *)  bits;
         fds.out     = (unsigned long *) (bits +   size);
         fds.ex      = (unsigned long *) (bits + 2*size);
@@ -1600,7 +1604,8 @@ int compat_core_sys_select(int n, compat_ulong_t __user *inp,
             compat_set_fd_set(n, exp, fds.res_ex))
                 ret = -EFAULT;
 out:
-        kfree(bits);
+        if (bits != stack_fds)
+                kfree(bits);
 out_nofds:
         return ret;
 }