Re: [RFC][PATCH] make global bitlock waitqueues per-node

[Dave Hansen]

On 12/19/2016 03:07 PM, Linus Torvalds wrote:
>     +wait_queue_head_t *bit_waitqueue(void *word, int bit)
>     +{
>     +       const int __maybe_unused nid = page_to_nid(virt_to_page(word));
>     +
>     +       return __bit_waitqueue(word, bit, nid);
> 
> No can do. Part of the problem with the old coffee was that it did that
> virt_to_page() crud. That doesn't work with the virtually mapped stack. 

Ahhh, got it.

So, what did you have in mind?  Just redirect bit_waitqueue() to the
"first_online_node" waitqueues?

wait_queue_head_t *bit_waitqueue(void *word, int bit)
{
        return __bit_waitqueue(word, bit, first_online_node);
}

We could do some fancy stuff like only do virt_to_page() for things in
the linear map, but I'm not sure we'll see much of a gain for it.  None
of the other waitqueue users look as pathological as the 'struct page'
ones.  Maybe:

wait_queue_head_t *bit_waitqueue(void *word, int bit)
{
        int nid
        if (word >= VMALLOC_START) /* all addrs not in linear map */
                nid = first_online_node;
        else
                nid = page_to_nid(virt_to_page(word));
        return __bit_waitqueue(word, bit, nid);
}