Minchan Kim <minc...@kernel.org> writes: > On Thu, Dec 21, 2017 at 03:48:56PM +0800, Huang, Ying wrote: >> Minchan Kim <minc...@kernel.org> writes: >> >> > On Wed, Dec 20, 2017 at 09:26:32AM +0800, Huang, Ying wrote: >> >> From: Huang Ying <ying.hu...@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 <hu...@google.com> >> >> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> >> >> Cc: Minchan Kim <minc...@kernel.org> >> >> Cc: Johannes Weiner <han...@cmpxchg.org> >> >> Cc: Tim Chen <tim.c.c...@linux.intel.com> >> >> Cc: Shaohua Li <s...@fb.com> >> >> Cc: Mel Gorman <mgor...@techsingularity.net> >> >> Cc: "Jrme Glisse" <jgli...@redhat.com> >> >> Cc: Michal Hocko <mho...@suse.com> >> >> Cc: Andrea Arcangeli <aarca...@redhat.com> >> >> Cc: David Rientjes <rient...@google.com> >> >> Cc: Rik van Riel <r...@redhat.com> >> >> Cc: Jan Kara <j...@suse.cz> >> >> Cc: Dave Jiang <dave.ji...@intel.com> >> >> Cc: Aaron Lu <aaron...@intel.com> >> >> Signed-off-by: "Huang, Ying" <ying.hu...@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. > 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
