• Huang Ying's avatar
    mm, swap: fix race between swap count continuation operations · 2628bd6f
    Huang Ying authored
    One page may store a set of entries of the sis->swap_map
    (swap_info_struct->swap_map) in multiple swap clusters.
    
    If some of the entries has sis->swap_map[offset] > SWAP_MAP_MAX,
    multiple pages will be used to store the set of entries of the
    sis->swap_map.  And the pages are linked with page->lru.  This is called
    swap count continuation.  To access the pages which store the set of
    entries of the sis->swap_map simultaneously, previously, sis->lock is
    used.  But to improve the scalability of __swap_duplicate(), swap
    cluster lock may be used in swap_count_continued() now.  This may race
    with add_swap_count_continuation() which operates on a nearby swap
    cluster, in which the sis->swap_map entries are stored in the same page.
    
    The race can cause wrong swap count in practice, thus cause unfreeable
    swap entries or software lockup, etc.
    
    To fix the race, a new spin lock called cont_lock is added to struct
    swap_info_struct to protect the swap count continuation page list.  This
    is a lock at the swap device level, so the scalability isn't very well.
    But it is still much better than the original sis->lock, because it is
    only acquired/released when swap count continuation is used.  Which is
    considered rare in practice.  If it turns out that the scalability
    becomes an issue for some workloads, we can split the lock into some
    more fine grained locks.
    
    Link: http://lkml.kernel.org/r/20171017081320.28133-1-ying.huang@intel.com
    Fixes: 235b6217 ("mm/swap: add cluster lock")
    Signed-off-by: default avatar"Huang, Ying" <ying.huang@intel.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Tim Chen <tim.c.chen@intel.com>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Aaron Lu <aaron.lu@intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: <stable@vger.kernel.org>	[4.11+]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    2628bd6f
swapfile.c 94.5 KB