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Message-Id: <20140509125426.e585b751a81bfcc96b6b8b88@linux-foundation.org>
Date: Fri, 9 May 2014 12:54:26 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jianyu Zhan <nasa4836@...il.com>
Cc: cl@...ux.com, mhocko@...e.cz, hannes@...xchg.org, riel@...hat.com,
minchan@...nel.org, zhangyanfei@...fujitsu.com,
hanpt@...ux.vnet.ibm.com, sasha.levin@...cle.com, oleg@...hat.com,
fabf@...net.be, mgorman@...e.de, aarcange@...hat.com,
cldu@...vell.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use a irq-safe __mod_zone_page_state in
mlocked_vma_newpage()
On Sat, 10 May 2014 00:16:48 +0800 Jianyu Zhan <nasa4836@...il.com> wrote:
> mlocked_vma_newpage() is only called in fault path by
> page_add_new_anon_rmap(), which is called on a *new* page.
> And such page is initially only visible via the pagetables, and the
> pte is locked while calling page_add_new_anon_rmap(), so we need not
> use an irq-safe mod_zone_page_state() here, using a light-weight version
> __mod_zone_page_state() would be OK.
>
> ...
>
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -196,7 +196,7 @@ static inline int mlocked_vma_newpage(struct vm_area_struct *vma,
> return 0;
>
> if (!TestSetPageMlocked(page)) {
> - mod_zone_page_state(page_zone(page), NR_MLOCK,
> + __mod_zone_page_state(page_zone(page), NR_MLOCK,
> hpage_nr_pages(page));
> count_vm_event(UNEVICTABLE_PGMLOCKED);
> }
The comment over __mod_zone_page_state() says "For use when we know
that interrupts are disabled". But that is not the case here.
Please fix this up, probably by altering the __mod_zone_page_state()
documentation.
Your proposed change to (the very poorly named) mlocked_vma_newpage()
makes that function more dangerous - people might add new callsites
without knowing that they're adding races. This risk can be reduced by
a) adding documentation explaining why __mod_zone_page_state() is
used and why it is safe and
b) moving mlocked_vma_newpage() out of internal.h and placing it in
mm/rmap.c immediately before page_add_new_anon_rmap(). Or, probably
better, by removing mlocked_vma_newpage() altogether and open-coding
its logic into page_add_new_anon_rmap().
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