linux-kernel - [3.7-rc6] capture_free_page() frees page without accounting for them??

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Message-ID: <50ABE741.2020604@linux.vnet.ibm.com>
Date:	Tue, 20 Nov 2012 12:25:37 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	linux-mm@...ck.org, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: [3.7-rc6] capture_free_page() frees page without accounting for them??

Hi Mel,

I'm chasing an apparent memory leak introduced post-3.6.  The
interesting thing is that it appears that the pages are in the
allocator, but not being accounted for:

	http://www.spinics.net/lists/linux-mm/msg46187.html
	https://bugzilla.kernel.org/show_bug.cgi?id=50181

I started auditing anything that might be messing with NR_FREE_PAGES,
and came across commit 1fb3f8ca.  It does something curious with
capture_free_page() (previously known as split_free_page()).

int capture_free_page(struct page *page, int alloc_order,
...
        __mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order));

-       /* Split into individual pages */
-       set_page_refcounted(page);
-       split_page(page, order);
+       if (alloc_order != order)
+               expand(zone, page, alloc_order, order,
+                       &zone->free_area[order], migratetype);

Note that expand() puts the pages _back_ in the allocator, but it does
not bump NR_FREE_PAGES.  We "return" alloc_order' worth of pages, but we
accounted for removing 'order'.

I _think_ the correct fix is to just:

-     __mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order));
+     __mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << alloc_order));

I'm trying to confirm the theory my making this happen a bit more often,
but I'd appreciate a second pair of eyes on the code in case I'm reading
it wrong.

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