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

[Matthew Wilcox]

On Mon, Apr 05, 2021 at 02:01:58PM -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.

Hi!  Current closest-thing-we-have-to-an-expert-on-struct-page here!

I haven't read over these patches yet.  If these pages are in use by
vmalloc, they can't use mapping+index because get_user_pages() will call
page_mapping() and the list_head will confuse it.  I think it could use
index+private for a list_head.

If the pages are in the buddy, I _think_ mapping+index are free.  private
is in use for buddy order.  But I haven't read through the buddy code
in a while.

Does it need to be a doubly linked list?  Can it be an hlist?