Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes: > On Wed, 7 Feb 2018 15:00:35 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> 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. >> >> ... >> >> --- 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