From: Huang Ying huang.ying.caritas@gmail.com Subject: mm, swap, frontswap: fix THP swap if frontswap enabled

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 bd4c82c22c367e068 ("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: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out") Signed-off-by: "Huang, Ying" ying.huang@intel.com Reported-by: Sergey Senozhatsky sergey.senozhatsky@gmail.com Tested-by: Sergey 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: Andrew Morton akpm@linux-foundation.org ---

drivers/xen/tmem.c | 4 ++++ mm/zswap.c | 6 ++++++ 2 files changed, 10 insertions(+)

diff -puN drivers/xen/tmem.c~mm-swap-frontswap-fix-thp-swap-if-frontswap-enabled-v3 drivers/xen/tmem.c --- a/drivers/xen/tmem.c~mm-swap-frontswap-fix-thp-swap-if-frontswap-enabled-v3 +++ a/drivers/xen/tmem.c @@ -284,6 +284,10 @@ static int tmem_frontswap_store(unsigned int pool = tmem_frontswap_poolid; int ret;

+ /* THP isn't supported */ + if (PageTransHuge(page)) + return -1; + if (pool < 0) return -1; if (ind64 != ind) diff -puN mm/zswap.c~mm-swap-frontswap-fix-thp-swap-if-frontswap-enabled-v3 mm/zswap.c --- a/mm/zswap.c~mm-swap-frontswap-fix-thp-swap-if-frontswap-enabled-v3 +++ a/mm/zswap.c @@ -1007,6 +1007,12 @@ static int zswap_frontswap_store(unsigne u8 *src, *dst; struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };

+ /* THP isn't supported */ + if (PageTransHuge(page)) { + ret = -EINVAL; + goto reject; + } + if (!zswap_enabled || !tree) { ret = -ENODEV; goto reject; _