On Wed, Feb 07, 2018 at 03:00:35PM +0800, Huang, Ying wrote:
> From: Huang Ying <huang.ying.caritas@xxxxxxxxx>
>
> 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 splitting THP before writing the page to swap device
> if frontswap is enabled. To deal with the situation where frontswap
> is enabled at runtime, whether the page is THP is checked before using
> frontswap during swapping out too.
>
> Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: Dan Streetman <ddstreet@xxxxxxxx>
> Cc: Seth Jennings <sjenning@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 4.14
> Fixes: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out")
>
> Changelog:
>
> v2:
>
> - Move frontswap check into swapfile.c to avoid to make vmscan.c
> depends on frontswap.
> ---
> mm/page_io.c | 2 +-
> mm/swapfile.c | 3 +++
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index b41cf9644585..6dca817ae7a0 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -250,7 +250,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> unlock_page(page);
> goto out;
> }
> - if (frontswap_store(page) == 0) {
> + if (!PageTransHuge(page) && frontswap_store(page) == 0) {
Why do we need this?
If frontswap_enabled is enabled but it doesn't support THP, it doesn't allow
cluster allocation by below logic so any THP page shouldn't come this path.
What do I missing now?
> set_page_writeback(page);
> unlock_page(page);
> end_page_writeback(page);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 006047b16814..0b7c7883ce64 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -934,6 +934,9 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
>
> /* Only single cluster request supported */
> WARN_ON_ONCE(n_goal > 1 && cluster);
> + /* Frontswap doesn't support THP */
> + if (frontswap_enabled() && cluster)
> + goto noswap;
>
> avail_pgs = atomic_long_read(&nr_swap_pages) / nr_pages;
> if (avail_pgs <= 0)
> --
> 2.15.1
>