• Huang Ying's avatar
    mm, swap, frontswap: fix THP swap if frontswap enabled · 62160e34
    Huang Ying authored
    commit 7ba71669 upstream.
    
    It was reported by Sergey Senozhatsky that if THP (Transparent Huge
    Page) and frontswap (via zswap) are both enabled, when memory goes low
    so that swap is triggered, segfault and memory corruption will occur in
    random user space applications as follow,
    
    kernel: urxvt[338]: segfault at 20 ip 00007fc08889ae0d sp 00007ffc73a7fc40 error 6 in libc-2.26.so[7fc08881a000+1ae000]
     #0  0x00007fc08889ae0d _int_malloc (libc.so.6)
     #1  0x00007fc08889c2f3 malloc (libc.so.6)
     #2  0x0000560e6004bff7 _Z14rxvt_wcstoutf8PKwi (urxvt)
     #3  0x0000560e6005e75c n/a (urxvt)
     #4  0x0000560e6007d9f1 _ZN16rxvt_perl_interp6invokeEP9rxvt_term9hook_typez (urxvt)
     #5  0x0000560e6003d988 _ZN9rxvt_term9cmd_parseEv (urxvt)
     #6  0x0000560e60042804 _ZN9rxvt_term6pty_cbERN2ev2ioEi (urxvt)
     #7  0x0000560e6005c10f _Z17ev_invoke_pendingv (urxvt)
     #8  0x0000560e6005cb55 ev_run (urxvt)
     #9  0x0000560e6003b9b9 main (urxvt)
     #10 0x00007fc08883af4a __libc_start_main (libc.so.6)
     #11 0x0000560e6003f9da _start (urxvt)
    
    After bisection, it was found the first bad commit is bd4c82c2 ("mm,
    THP, swap: delay splitting THP after swapped out").
    
    The root cause is as follows:
    
    When the pages are written to swap device during swapping out in
    swap_writepage(), zswap (fontswap) is tried to compress the pages to
    improve performance.  But zswap (frontswap) will treat THP as a normal
    page, so only the head page is saved.  After swapping in, tail pages
    will not be restored to their original contents, causing memory
    corruption in the applications.
    
    This is fixed by refusing to save page in the frontswap store functions
    if the page is a THP.  So that the THP will be swapped out to swap
    device.
    
    Another choice is to split THP if frontswap is enabled.  But it is found
    that the frontswap enabling isn't flexible.  For example, if
    CONFIG_ZSWAP=y (cannot be module), frontswap will be enabled even if
    zswap itself isn't enabled.
    
    Frontswap has multiple backends, to make it easy for one backend to
    enable THP support, the THP checking is put in backend frontswap store
    functions instead of the general interfaces.
    
    Link: http://lkml.kernel.org/r/20180209084947.22749-1-ying.huang@intel.com
    Fixes: bd4c82c2 ("mm, THP, swap: delay splitting THP after swapped out")
    Signed-off-by: default avatar"Huang, Ying" <ying.huang@intel.com>
    Reported-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Tested-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Suggested-by: Minchan Kim <minchan@kernel.org>	[put THP checking in backend]
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Cc: Dan Streetman <ddstreet@ieee.org>
    Cc: Seth Jennings <sjenning@redhat.com>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Mel Gorman <mgorman@techsingularity.net>
    Cc: Shakeel Butt <shakeelb@google.com>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: <stable@vger.kernel.org>	[4.14]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    62160e34