Re: [patch 5/8] mm: memcg: remove unneeded checks from newpage_charge()

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

On Thu, Nov 24, 2011 at 09:04:43AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 23 Nov 2011 16:42:28 +0100
> Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org> wrote:
> 
> > From: Johannes Weiner <jweiner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> > 
> > All callsites pass in freshly allocated pages and a valid mm.  As a
> > result, all checks pertaining the page's mapcount, page->mapping or
> > the fallback to init_mm are unneeded.
> > 
> > Signed-off-by: Johannes Weiner <jweiner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> 
> Hmm, it's true now. But for making clear our assumption to all readers of code,
> 
> could you add
> VM_BUG_ON(!mm || page_mapped(page) || (page->mapping && !PageAnon(page)) ?

Of course.  Please find the delta patch below.  I broke them up into
three separate checks to make the problem easier to find if the BUG
triggers.

> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>

Thank you.

---
From: Johannes Weiner <jweiner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: mm: memcg: remove unneeded checks from newpage_charge() fix

Document page state assumptions and catch if they are violated in
newpage_charge().

Signed-off-by: Johannes Weiner <jweiner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 mm/memcontrol.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0d10be4..f338018 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2679,6 +2679,9 @@ int mem_cgroup_newpage_charge(struct page *page,
 {
 	if (mem_cgroup_disabled())
 		return 0;
+	VM_BUG_ON(page_mapped(page));
+	VM_BUG_ON(page->mapping && !PageAnon(page));
+	VM_BUG_ON(!mm);
 	return mem_cgroup_charge_common(page, mm, gfp_mask,
 					MEM_CGROUP_CHARGE_TYPE_MAPPED);
 }
-- 
1.7.6.4
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