From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: "Minchan Kim" <minchan@kernel.org>,
"Andrew Morton" <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Hugh Dickins" <hughd@google.com>,
"Johannes Weiner" <hannes@cmpxchg.org>,
"Tim Chen" <tim.c.chen@linux.intel.com>,
"Shaohua Li" <shli@fb.com>,
"Mel Gorman" <mgorman@techsingularity.net>,
"J�r�me Glisse" <jglisse@redhat.com>,
"Michal Hocko" <mhocko@suse.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"David Rientjes" <rientjes@google.com>,
"Rik van Riel" <riel@redhat.com>, "Jan Kara" <jack@suse.cz>,
"Dave Jiang" <dave.jiang@intel.com>,
"Aaron Lu" <aaron.lu@intel.com>
Subject: Re: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations
Date: Fri, 22 Dec 2017 08:14:47 -0800 [thread overview]
Message-ID: <20171222161447.GF7829@linux.vnet.ibm.com> (raw)
In-Reply-To: <87r2rmj1d8.fsf@yhuang-dev.intel.com>
On Fri, Dec 22, 2017 at 10:14:43PM +0800, Huang, Ying wrote:
> Minchan Kim <minchan@kernel.org> writes:
>
> > On Thu, Dec 21, 2017 at 03:48:56PM +0800, Huang, Ying wrote:
> >> Minchan Kim <minchan@kernel.org> writes:
> >>
> >> > On Wed, Dec 20, 2017 at 09:26:32AM +0800, Huang, Ying wrote:
> >> >> From: Huang Ying <ying.huang@intel.com>
> >> >>
> >> >> When the swapin is performed, after getting the swap entry information
> >> >> from the page table, system will swap in the swap entry, without any
> >> >> lock held to prevent the swap device from being swapoff. This may
> >> >> cause the race like below,
> >> >>
> >> >> CPU 1 CPU 2
> >> >> ----- -----
> >> >> do_swap_page
> >> >> swapin_readahead
> >> >> __read_swap_cache_async
> >> >> swapoff swapcache_prepare
> >> >> p->swap_map = NULL __swap_duplicate
> >> >> p->swap_map[?] /* !!! NULL pointer access */
> >> >>
> >> >> Because swapoff is usually done when system shutdown only, the race
> >> >> may not hit many people in practice. But it is still a race need to
> >> >> be fixed.
> >> >>
> >> >> To fix the race, get_swap_device() is added to check whether the
> >> >> specified swap entry is valid in its swap device. If so, it will keep
> >> >> the swap entry valid via preventing the swap device from being
> >> >> swapoff, until put_swap_device() is called.
> >> >>
> >> >> Because swapoff() is very race code path, to make the normal path runs
> >> >> as fast as possible, RCU instead of reference count is used to
> >> >> implement get/put_swap_device(). From get_swap_device() to
> >> >> put_swap_device(), the RCU read lock is held, so synchronize_rcu() in
> >> >> swapoff() will wait until put_swap_device() is called.
> >> >>
> >> >> In addition to swap_map, cluster_info, etc. data structure in the
> >> >> struct swap_info_struct, the swap cache radix tree will be freed after
> >> >> swapoff, so this patch fixes the race between swap cache looking up
> >> >> and swapoff too.
> >> >>
> >> >> Cc: Hugh Dickins <hughd@google.com>
> >> >> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >> >> Cc: Minchan Kim <minchan@kernel.org>
> >> >> Cc: Johannes Weiner <hannes@cmpxchg.org>
> >> >> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> >> >> Cc: Shaohua Li <shli@fb.com>
> >> >> Cc: Mel Gorman <mgorman@techsingularity.net>
> >> >> Cc: "Jrme Glisse" <jglisse@redhat.com>
> >> >> Cc: Michal Hocko <mhocko@suse.com>
> >> >> Cc: Andrea Arcangeli <aarcange@redhat.com>
> >> >> Cc: David Rientjes <rientjes@google.com>
> >> >> Cc: Rik van Riel <riel@redhat.com>
> >> >> Cc: Jan Kara <jack@suse.cz>
> >> >> Cc: Dave Jiang <dave.jiang@intel.com>
> >> >> Cc: Aaron Lu <aaron.lu@intel.com>
> >> >> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> >> >>
> >> >> Changelog:
> >> >>
> >> >> v4:
> >> >>
> >> >> - Use synchronize_rcu() in enable_swap_info() to reduce overhead of
> >> >> normal paths further.
> >> >
> >> > Hi Huang,
> >>
> >> Hi, Minchan,
> >>
> >> > This version is much better than old. To me, it's due to not rcu,
> >> > srcu, refcount thing but it adds swap device dependency(i.e., get/put)
> >> > into every swap related functions so users who don't interested on swap
> >> > don't need to care of it. Good.
> >> >
> >> > The problem is caused by freeing by swap related-data structure
> >> > *dynamically* while old swap logic was based on static data
> >> > structure(i.e., never freed and the verify it's stale).
> >> > So, I reviewed some places where use PageSwapCache and swp_entry_t
> >> > which could make access of swap related data structures.
> >> >
> >> > A example is __isolate_lru_page
> >> >
> >> > It calls page_mapping to get a address_space.
> >> > What happens if the page is on SwapCache and raced with swapoff?
> >> > The mapping got could be disappeared by the race. Right?
> >>
> >> Yes. We should think about that. Considering the file cache pages, the
> >> address_space backing the file cache pages may be freed dynamically too.
> >> So to use page_mapping() return value for the file cache pages, some
> >> kind of locking is needed to guarantee the address_space isn't freed
> >> under us. Page may be locked, or under writeback, or some other locks
> >
> > I didn't look at the code in detail but I guess every file page should
> > be freed before the address space destruction and page_lock/lru_lock makes
> > the work safe, I guess. So, it wouldn't be a problem.
> >
> > However, in case of swapoff, it doesn't remove pages from LRU list
> > so there is no lock to prevent the race at this moment. :(
>
> Take a look at file cache pages and file cache address_space freeing
> code path. It appears that similar situation is possible for them too.
>
> The file cache pages will be delete from file cache address_space before
> address_space (embedded in inode) is freed. But they will be deleted
> from LRU list only when its refcount dropped to zero, please take a look
> at put_page() and release_pages(). While address_space will be freed
> after putting reference to all file cache pages. If someone holds a
> reference to a file cache page for quite long time, it is possible for a
> file cache page to be in LRU list after the inode/address_space is
> freed.
>
> And I found inode/address_space is freed witch call_rcu(). I don't know
> whether this is related to page_mapping().
>
> This is just my understanding.
>
> >> need to be held, for example, page table lock, or lru_lock, etc. For
> >> __isolate_lru_page(), lru_lock will be held when it is called. And we
> >> will call synchronize_rcu() between clear PageSwapCache and free swap
> >> cache, so the usage of swap cache in __isolate_lru_page() should be
> >> safe. Do you think my analysis makes sense?
> >
> > I don't understand how synchronize_rcu closes the race with spin_lock.
> > Paul might help it.
>
> Per my understanding, spin_lock() will preempt_disable(), so
> synchronize_rcu() will wait until spin_unlock() is called.
Only when CONFIG_PREEMPT=n!
In CONFIG_PREEMPT=y kernels, preempt_disable() won't necessarily prevent
synchronize_rcu() from completing.
Now, preempt_disable() does prevent synchronize_sched() from
completing, but that would require changing the rcu_read_lock() and
rcu_read_unlock() to rcu_read_lock_sched()/rcu_read_unlock_sched()
or preempt_enable()/preempt_disable().
Another fix would be to invoke rcu_read_lock() just after acquiring
the spinlock and rcu_read_unlock() just before releasing it.
Thanx, Paul
> > Even if we solve it, there is a other problem I spot.
> > When I see migrate_vma_pages, it pass mapping to migrate_page which
> > accesses mapping->tree_lock unconditionally even though the address_space
> > is already gone.
>
> Before migrate_vma_pages() is called, migrate_vma_prepare() is called,
> where pages are locked. So it is safe.
>
> > Hmm, I didn't check all sites where uses PageSwapCache, swp_entry_t
> > but gut feeling is it would be not simple.
>
> Yes. We should check all sites. Thanks for your help!
>
> Best Regards,
> Huang, Ying
>
next prev parent reply other threads:[~2017-12-22 16:14 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-20 1:26 [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations Huang, Ying
2017-12-21 2:16 ` Minchan Kim
[not found] ` <871sjopllj.fsf@yhuang-dev.intel.com>
2017-12-21 23:58 ` Minchan Kim
2017-12-22 14:14 ` Huang, Ying
2017-12-22 16:14 ` Paul E. McKenney [this message]
2017-12-25 1:28 ` Huang, Ying
2017-12-23 1:36 ` Minchan Kim
2017-12-26 5:33 ` Huang, Ying
2018-01-02 10:21 ` Mel Gorman
2018-01-02 11:29 ` Jan Kara
2018-01-02 13:29 ` Mel Gorman
2018-01-03 0:42 ` Huang, Ying
2018-01-03 9:54 ` Mel Gorman
2018-01-04 1:17 ` Huang, Ying
2018-01-04 10:21 ` Mel Gorman
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