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Date: Mon, 11 May 2015 16:59:27 +0800
From: yalin wang <yalin.wang2010@...il.com>
To: Vladimir Davydov <vdavydov@...allels.com>
Cc: Rik van Riel <riel@...hat.com>, Hugh Dickins <hughd@...gle.com>,
Christoph Lameter <cl@...ux.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Minchan Kim <minchan@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...e.cz>,
Greg Thelen <gthelen@...gle.com>,
Michel Lespinasse <walken@...gle.com>,
David Rientjes <rientjes@...gle.com>,
Pavel Emelyanov <xemul@...allels.com>,
Cyrill Gorcunov <gorcunov@...nvz.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC] rmap: fix "race" between do_wp_page and shrink_active_list
2015-05-11 15:51 GMT+08:00 Vladimir Davydov <vdavydov@...allels.com>:
> Hi,
>
> I've been arguing with Minchan for a while about whether store-tearing
> is possible while setting page->mapping in __page_set_anon_rmap and
> friends, see
>
> http://thread.gmane.org/gmane.linux.kernel.mm/131949/focus=132132
>
> This patch is intended to draw attention to this discussion. It fixes a
> race that could happen if store-tearing were possible. The race is as
> follows.
>
> In do_wp_page() we can call page_move_anon_rmap(), which sets
> page->mapping as follows:
>
> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> page->mapping = (struct address_space *) anon_vma;
>
> The page in question may be on an LRU list, because nowhere in
> do_wp_page() we remove it from the list, neither do we take any LRU
> related locks. Although the page is locked, shrink_active_list() can
> still call page_referenced() on it concurrently, because the latter does
> not require an anonymous page to be locked.
>
> If store tearing described in the thread were possible, we could face
> the following race resulting in kernel panic:
>
> CPU0 CPU1
> ---- ----
> do_wp_page shrink_active_list
> lock_page page_referenced
> PageAnon->yes, so skip trylock_page
> page_move_anon_rmap
> page->mapping = anon_vma
> rmap_walk
> PageAnon->no
> rmap_walk_file
> BUG
> page->mapping += PAGE_MAPPING_ANON
>
> This patch fixes this race by explicitly forbidding the compiler to
> split page->mapping store in __page_set_anon_rmap() and friends and load
> in PageAnon() with the aid of WRITE/READ_ONCE.
>
> Personally, I don't believe that this can ever happen on any sane
> compiler, because such an "optimization" would only result in two stores
> vs one (note, anon_vma is not a constant), but since I can be mistaken I
> would like to hear from synchronization experts what they think about
> it.
>
> Thanks,
> Vladimir
> ---
> include/linux/page-flags.h | 3 ++-
> mm/rmap.c | 6 +++---
> 2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5e7c4f50a644..a529e0a35fe9 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -320,7 +320,8 @@ PAGEFLAG(Idle, idle)
>
> static inline int PageAnon(struct page *page)
> {
> - return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
> + return ((unsigned long)READ_ONCE(page->mapping) &
> + PAGE_MAPPING_ANON) != 0;
> }
>
> #ifdef CONFIG_KSM
> diff --git a/mm/rmap.c b/mm/rmap.c
> index eca7416f55d7..aa60c63704e6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -958,7 +958,7 @@ void page_move_anon_rmap(struct page *page,
> VM_BUG_ON_PAGE(page->index != linear_page_index(vma, address), page);
>
> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> - page->mapping = (struct address_space *) anon_vma;
> + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
> }
>
> /**
> @@ -987,7 +987,7 @@ static void __page_set_anon_rmap(struct page *page,
> anon_vma = anon_vma->root;
>
> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> - page->mapping = (struct address_space *) anon_vma;
> + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
> page->index = linear_page_index(vma, address);
> }
>
> @@ -1579,7 +1579,7 @@ static void __hugepage_set_anon_rmap(struct page *page,
> anon_vma = anon_vma->root;
>
> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> - page->mapping = (struct address_space *) anon_vma;
> + WRITE_ONCE(page->mapping, (struct address_space *) anon_vma);
> page->index = linear_page_index(vma, address);
> }
>
> --
> 1.7.10.4
>
> --
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i am confused about your analysis ,
for the race stack:
CPU0 CPU1
---- ----
do_wp_page shrink_active_list
lock_page page_referenced
PageAnon->yes, so skip trylock_page
page_move_anon_rmap
page->mapping = anon_vma
rmap_walk
PageAnon->no
rmap_walk_file
BUG
page->mapping += PAGE_MAPPING_ANON
the page should must change from PageAnon() to !PageAnon() when crash happened.
but page_move_anon_rmap() is doing change a page from !PageAnon()
(swapcache page)
to PageAnon() , how does this race condition crash happened ?
BRs,
Yalin
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