Re: put_page() tries to handle hugepages but fails

[William Lee Irwin III]

On Fri, Apr 23, 2004 at 01:34:37AM -0700, Andrew Morton wrote:
> We could certainly remove the test for a null destructor in there and
> require that compound pages have a destructor installed.
> But the main reason why that code is in there is for transparently handling
> direct-io into hugepage regions.  That code does perform put_page against
> 4k pageframes within the huge page and it does follow the pointer to the
> head page.
> With your patch applied get_user_pages() and bio_release_pages() will
> manipulate the refcounts of the inner 4k pages rather than the head pages
> and things will explode.
> We could change follow_hugetlb_page() to always take a ref against the head
> page and we could teach bio_release_pages() to perform appropriate pfn
> masking to locate the head page, and perform similar tricks for
> futexes-in-large-pages.  But with the code as-is the refcounting works
> transparently.
> If it's "broken" I wanna know why.

The destructor is never invoked from that path, but it's not the path that
should free it anyway. To me it appears the call to __page_cache_release()
on the head of the hugepage should just be removed in favor of doing
nothing; at best it can only race against concurrent put_page(), see
page_count(page) vanish, and accidentally call free_hot_page() against
the head of the hugepage. As hugepages are never on the LRU, the
remainder of __page_cache_release() should be a nop for them.

Untested patch below.


-- wli


Index: wli-2.6.6-rc2-mm1/mm/swap.c
===================================================================
--- wli-2.6.6-rc2-mm1.orig/mm/swap.c	2004-04-21 05:19:58.000000000 -0700
+++ wli-2.6.6-rc2-mm1/mm/swap.c	2004-04-23 03:21:22.000000000 -0700
@@ -41,15 +41,12 @@
 	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);
-			}
+			void (*destructor)(struct page *);
+			destructor = (void (*)(struct page *))page[1].mapping;
+			BUG_ON(!destructor);
+			(*destructor)(page);
 		}
-		return;
-	}
-	if (!PageReserved(page) && put_page_testzero(page))
+	} else if (!PageReserved(page) && put_page_testzero(page))
 		__page_cache_release(page);
 }
 EXPORT_SYMBOL(put_page);
-
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