Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled

[huang ying]

On Thu, Feb 8, 2018 at 6:17 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 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?

If frontswap_enabled() becomes true at runtime after swap cluster
allocation but before swap_writepage(), this can prevent memory
corruption.  I know this isn't true now.  Because frontswap isn't very
dynamic now.  But I still think this is good thing to do.

Best Regards,
Huang, Ying

>>               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
>>