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SubjectRe: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled
Date
Andrew Morton <akpm@linux-foundation.org> writes:

> On Wed, 7 Feb 2018 15:00:35 +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 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.
>>
>> ...
>>
>> --- 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) {
>> 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;
>>
>
> hm. This is assuming that "cluster==true" means "this is thp swap".
> That's presently true, but is it appropriate that get_swap_pages() is
> peeking at "cluster" to work out why it is being called?
>
> Or would it be cleaner to do this in get_swap_page()? Something like
>
> --- a/mm/swap_slots.c~a
> +++ a/mm/swap_slots.c
> @@ -317,8 +317,11 @@ swp_entry_t get_swap_page(struct page *p
> entry.val = 0;
>
> if (PageTransHuge(page)) {
> - if (IS_ENABLED(CONFIG_THP_SWAP))
> - get_swap_pages(1, true, &entry);
> + /* Frontswap doesn't support THP */
> + if (!frontswap_enabled()) {
> + if (IS_ENABLED(CONFIG_THP_SWAP))
> + get_swap_pages(1, true, &entry);
> + }
> return entry;
> }
>

Sure. I will do this.

Best Regards,
Huang, Ying

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