Subject: Re: select(0, ..) is valid ?

From: Nishanth Aravamudan <[email-masked]>
Date: Tue, 22 May 2007 07:34:32 -0700

On 22.05.2007 [09:16:37 -0500], Steve Fox wrote:
> On Wed, 2007-05-16 at 17:59 -0700, Badari Pulavarty wrote:
> 
> > 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 <[email-masked]>
> > ---
> >  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;
> >  }
> 
> Andy put this through a couple machines on test.kernel.org and elm3b6
> was fixed, however elm3b239 still had a boot error.
> 
> BUG: at mm/slab.c:777 __find_general_cachep()
> 
> Call Trace:
>  [<ffffffff802729c6>] __kmalloc+0xa6/0xe0
>  [<ffffffff8021d21b>] cache_k8_northbridges+0x9b/0x120

I believe this is fixed by:

http://lkml.org/lkml/2007/5/18/19

Care to stack it on top and retest?

Thanks,
Nish

-- 
Nishanth Aravamudan <[email-masked]>
IBM Linux Technology Center