Re: [PATCH -V4 -mm] mm, swap: Fix race between swapoff and some swap operations

[Paul E. McKenney]

On Fri, Dec 22, 2017 at 10:14:43PM +0800, Huang, Ying wrote:
> Minchan Kim <minchan@xxxxxxxxxx> writes:
> 
> > On Thu, Dec 21, 2017 at 03:48:56PM +0800, Huang, Ying wrote:
> >> Minchan Kim <minchan@xxxxxxxxxx> writes:
> >> 
> >> > On Wed, Dec 20, 2017 at 09:26:32AM +0800, Huang, Ying wrote:
> >> >> From: Huang Ying <ying.huang@xxxxxxxxx>
> >> >> 
> >> >> 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@xxxxxxxxxx>
> >> >> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> >> >> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> >> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> >> >> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >> >> Cc: Shaohua Li <shli@xxxxxx>
> >> >> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> >> >> Cc: "Jrme Glisse" <jglisse@xxxxxxxxxx>
> >> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
> >> >> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> >> >> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >> >> Cc: Rik van Riel <riel@xxxxxxxxxx>
> >> >> Cc: Jan Kara <jack@xxxxxxx>
> >> >> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> >> >> Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
> >> >> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> >> >> 
> >> >> 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
>