Re: select(0, ..) is valid ?
From: Steve Fox
Date: Tue May 22 2007 - 10:17:18 EST
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 <pbadari@xxxxxxxxxx>
> ---
> 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
[<ffffffff80688af3>] gart_iommu_init+0x33/0x5b0
[<ffffffff802211a3>] __wake_up+0x43/0x70
[<ffffffff80453b90>] genl_rcv+0x0/0x70
[<ffffffff80452175>] netlink_kernel_create+0x155/0x170
[<ffffffff80684029>] pci_iommu_init+0x9/0x20
[<ffffffff8067e6f4>] kernel_init+0x154/0x330
[<ffffffff8020a8d8>] child_rip+0xa/0x12
[<ffffffff80348e10>] acpi_ds_init_one_object+0x0/0x7c
[<ffffffff8067e5a0>] kernel_init+0x0/0x330
[<ffffffff8020a8ce>] child_rip+0x0/0x12
See the "2.6.22-rc2-git1 +1" row at http://test.kernel.org/ for full
logs.
--
Steve Fox
IBM Linux Technology Center
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