Dave Hansen wrote:

On Fri, 2004-08-20 at 19:31, Hiroyuki KAMEZAWA wrote: 

quoted

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 [off-list ref]

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