Minchan Kim <minchan@xxxxxxxxxx> 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@xxxxxxxxxx> wrote: >> > On Tue, Feb 06, 2018 at 04:39:18PM +0800, Huang, Ying wrote: >> >> Hi, Minchan, >> >> >> >> Minchan Kim <minchan@xxxxxxxxxx> writes: >> >> >> >> > Hi Huang, >> >> > >> >> > On Tue, Feb 06, 2018 at 02:54:04PM +0800, Huang, Ying wrote: >> >> >> From: Huang Ying <ying.huang@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 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