Re: [RFC 2/3] vmalloc: Support grouped page allocations

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Mon, 2021-04-05 at 14:01 -0700, Dave Hansen wrote:
> On 4/5/21 1:37 PM, Rick Edgecombe wrote:
> > +static void __dispose_pages(struct list_head *head)
> > +{
> > +       struct list_head *cur, *next;
> > +
> > +       list_for_each_safe(cur, next, head) {
> > +               list_del(cur);
> > +
> > +               /* The list head is stored at the start of the page
> > */
> > +               free_page((unsigned long)cur);
> > +       }
> > +}
> 
> This is interesting.
> 
> While the page is in the allocator, you're using the page contents
> themselves to store the list_head.  It took me a minute to figure out
> what you were doing here because: "start of the page" is a bit
> ambiguous.  It could mean:
> 
>  * the first 16 bytes in 'struct page'
> or
>  * the first 16 bytes in the page itself, aka *page_address(page)
> 
> The fact that this doesn't work on higmem systems makes this an OK
> thing
> to do, but it is a bit weird.  It's also doubly susceptible to bugs
> where there's a page_to_virt() or virt_to_page() screwup.
> 
> I was *hoping* there was still sufficient space in 'struct page' for
> this second list_head in addition to page->lru.  I think there
> *should*
> be.  That would at least make this allocator a bit more "normal" in
> not
> caring about page contents while the page is free in the allocator. 
> If
> you were able to do that you could do things like kmemcheck or page
> alloc debugging while the page is in the allocator.
> 
> Anyway, I think I'd prefer that you *try* to use 'struct page' alone.
> But, if that doesn't work out, please comment the snot out of this
> thing
> because it _is_ weird.

Yes sorry, that deserved more explanation. I tried putting it in struct
page actually. The problem was list_lru automatically determines the
node id from the list_head provided to it via
page_to_nid(virt_to_page(head)). I guess it assumes the list_head is on
the actual item. I started adding another list_lru function that let
the node id be passed in separately, but I remembered this trick from
the deferred free list in vmalloc.

If this ever expands to handle direct map unmapped pages (which would
probably be the next step), the list_head will have to be moved out of
the actual page anyway. But in the meantime it resulted in the smallest
change.

I can try the other way if it's still too weird.