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SubjectRe: [PATCH -mm -v3] mm, swap, frontswap: Fix THP swap if frontswap enabled
On Fri,  9 Feb 2018 16:49:47 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:

> From: Huang Ying <huang.ying.caritas@gmail.com>
>
> 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 follow.
>
> When the pages are written to swap device during swapping out in
> swap_writepage(), zswap (fontswap) is tried to compress the pages
> instead to improve the performance. But zswap (frontswap) will treat
> THP as normal page, so only the head page is saved. After swapping
> in, tail pages will not be restored to its original contents, so cause
> the memory corruption in the applications.
>
> This is fixed via rejecting to save page in 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.
>
> Fixes: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out")
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

I don't think Sergey has tested this version and I suspect this is a
holdover from the earlier patch, so I'll remove this line.

> Suggested-by: Minchan Kim <minchan@kernel.org> # put THP checking in backend
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> 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

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