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SubjectRe: [PATCH -mm] mm, swap, frontswap: Fix THP swap if frontswap enabled
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Minchan Kim <minchan@kernel.org> writes:

> On Tue, Feb 06, 2018 at 09:34:44PM +0800, huang ying wrote:
>> On Tue, Feb 6, 2018 at 5:02 PM, Minchan Kim <minchan@kernel.org> wrote:
>> > On Tue, Feb 06, 2018 at 04:39:18PM +0800, Huang, Ying wrote:
>> >> Hi, Minchan,
>> >>
>> >> Minchan Kim <minchan@kernel.org> writes:
>> >>
>> >> > Hi Huang,
>> >> >
>> >> > On Tue, Feb 06, 2018 at 02:54:04PM +0800, Huang, Ying wrote:
>> >> >> From: Huang Ying <ying.huang@intel.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 storage 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 at the begin of swapping out if
>> >> >> frontswap is enabled. To avoid frontswap to be enabled at runtime,
>> >> >> whether the page is THP is checked before using frontswap during
>> >> >> swapping out too.
>> >> >
>> >> > Nice catch, Huang. However, before the adding a new dependency between
>> >> > frontswap and vmscan that I want to avoid if it is possible, let's think
>> >> > whether frontswap can support THP page or not.
>> >> > Can't we handle it with some loop to handle all of subpages of THP page?
>> >> > It might be not hard?
>> >>
>> >> Yes. That could be an optimization over this patch. This patch is just
>> >> a simple fix to make things work and be suitable for stable tree.
>> >
>> > Yub, it would be more complex than this patch. However, this patch introduces
>> > a new dependency to vmscan.c. IOW, we have been good without knowing frontswap
>> > in vmscan.c but from now on, we should be aware of that, which is unfortunate.
>> >
>> > Can't we simple do like that if you want to make it simple and rely on someone
>> > who makes frontswap THP-aware later?
>> >
>> > diff --git a/mm/swapfile.c b/mm/swapfile.c
>> > index 42fe5653814a..4bf1725407aa 100644
>> > --- a/mm/swapfile.c
>> > +++ b/mm/swapfile.c
>> > @@ -934,7 +934,11 @@ 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);
>> > +#ifdef CONFIG_FRONTSWAP
>> > + /* Now, frontswap doesn't support THP page */
>> > + if (frontswap_enabled() && cluster)
>> > + return;
>> > +#endif
>> > avail_pgs = atomic_long_read(&nr_swap_pages) / nr_pages;
>> > if (avail_pgs <= 0)
>> > goto noswap;
>> >
>>
>> This can avoid introduce dependency on frontswap in vmscan.c. But
>> IMHO it doesn't look like the right place to place the logic.
>> vmscan.c is the place we put policy to determine whether to split THP.
>
> It adds split policy in vmscan.c like you said.
>
> shrink_page_list already relies on swap_file.c to decide split a THP page.
> IOW, if a THP swap stuff is not avilable, split a thp.
> It's totally same logic. I don't see any difference at all.
>
> shrink_page_list:
>
> if (!add_to_swap(page)) {
> if (PageTransHuge(page))
> goto activate_locked;
> if (split_huge_page_to_list(page, page_list))
> goto activate_locked;
> count_vm_event(THP_SWPOUT_FALLBACK);
> if (!add_to_swap(page))
> goto activate_locked;
> }

OK. I will change the code as you suggested.

Best Regards,
Huang, Ying

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