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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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