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SubjectRe: select(0, ..) is valid ?
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On Wed, 2007-05-16 at 10:37 -0500, Anton Blanchard wrote:
> Hi Hugh,
>
> > It's interesting that compat_core_sys_select() shows this kmalloc(0)
> > failure but core_sys_select() does not. That's because core_sys_select()
> > avoids kmalloc by using a buffer on the stack for small allocations (and
> > 0 sure is small). Shouldn't compat_core_sys_select() do just the same?
> > Or is SLUB going to be so efficient that doing so is a waste of time?
>
> Nice catch, the original optimisation from Andi is:
>
> http://git.kernel.org/git-new/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=70674f95c0a2ea694d5c39f4e514f538a09be36f
>
> And I think it makes sense for the compat code to do it too.
>
> Anton

Here it is ..

Should I do one for poll() also ?

Thanks,
Badari

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>
---
fs/compat.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)
Index: linux-2.6.22-rc1/fs/compat.c
===================================================================
--- linux-2.6.22-rc1.orig/fs/compat.c 2007-05-12 18:45:56.000000000 -0700
+++ linux-2.6.22-rc1/fs/compat.c 2007-05-16 17:50:39.000000000 -0700
@@ -1544,9 +1544,10 @@ int compat_core_sys_select(int n, compat
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
* 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
compat_set_fd_set(n, exp, fds.res_ex))
ret = -EFAULT;
out:
- kfree(bits);
+ if (bits != stack_fds)
+ kfree(bits);
out_nofds:
return ret;
}


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