Re: put_page() tries to handle hugepages but fails

[David Gibson <david@gibson.dropbear.id.au>]

On Mon, Apr 26, 2004 at 09:47:57PM -0700, Andrew Morton wrote:
> David Gibson <david@gibson.dropbear.id.au> wrote:
> >
> > +		if (put_page_testzero(page))
> >  +			free_huge_page(page);
> 
> Well yes, but this is assuming that compound pages are always hugetlb pages.
> 
> It's true at present, but it doesn't have to always be true.  The cost of
> the destructor is zilch, so why not?

True enough, I guess I was just following the "don't build it till you
need it" philosophy.

> Please review the changes which went into 2.6.6-rc2-mm2.

Sorry, should have done that earlier.  Looks reasonable with two small
exceptions: 1) put_page() can still theoretically call
__page_cache_release() which is wrong (and makes the code misleading)
- patch below replaces this with a BUG() if there is no destructor. 2)
what about wli's concern that mapping may be accessed without first
checking for a PageCompound?

Index: working-2.6/mm/swap.c
===================================================================
--- working-2.6.orig/mm/swap.c	2004-04-14 12:22:49.000000000 +1000
+++ working-2.6/mm/swap.c	2004-04-27 15:02:30.046342392 +1000
@@ -41,11 +41,9 @@
 	if (unlikely(PageCompound(page))) {
 		page = (struct page *)page->private;
 		if (put_page_testzero(page)) {
-			if (page[1].mapping) {	/* destructor? */
-				(*(void (*)(struct page *))page[1].mapping)(page);
-			} else {
-				__page_cache_release(page);
-			}
+			BUG_ON(! page[1].mapping);
+
+			(*(void (*)(struct page *))page[1].mapping)(page);
 		}
 		return;
 	}


-- 
David Gibson			| For every complex problem there is a
david AT gibson.dropbear.id.au	| solution which is simple, neat and
				| wrong.
http://www.ozlabs.org/people/dgibson