Re: [Lhms-devel] [RFC] free_area[] bitmap elimination [0/3]

Hiroyuki KAMEZAWA
2004-08-23T23:54:52Z

Dave Hansen wrote:
> On Fri, 2004-08-20 at 19:31, Hiroyuki KAMEZAWA wrote:
>
>>This patch removes bitmap from buddy allocator used in
>>alloc_pages()/free_pages() in the kernel 2.6.8.1.
>
>
> Looks very interesting. The most mysterious thing about it that I can
> think of right now would be its cache behavior. Since struct pages are
> at least 1/2 a cacheline on most architectures, you're going to dirty
> quite a few more cachelines than if you were accessing a quick bitmap.
> However, if the page was recently accessed you might get *better*
> cacheline performance because the struct page itself may have been
> hotter than its bitmap.
>

> The use of page_count()==0 is a little worrisome. There's almost
> certainly some race conditions where a page can be mistaken for free
> while it's page_count()==0, but before it's reached free_pages_bulk().
>
> BTW, even if page_count()==0 isn't a valid check like you fear, you
> could always steal a bit in the page->flags. Check out
> free_pages_check() in mm/page_alloc.c for a nice summary of what state
> pages have to be in before they're freed.
>
Thanks for your comment.

In this patch, "whether a page is free and in buddy allocator ?" is confirmed by
page_count(page) == 0 and page_order(page) == valid_order.
A valid_order is a value between (unsigned long)~0 - (unsigned long)~(MAX_ORDER)

But there may be pages which have vague page->private and conflict with my
buddy page checking.
I'd like to read free_pages_check() more and take page->flags into account.

-- KAME

--
--the clue is these footmarks leading to the door.--
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>