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

[Huang\, Ying]

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.

> 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