On Fri, 27 Sep 2019 15:47:03 -0400 Qian Cai <cai@xxxxxx> wrote: > On architectures like s390, arch_free_page() could mark the page unused > (set_page_unused()) and any access later would trigger a kernel panic. > Fix it by moving arch_free_page() after all possible accessing calls. > > Hardware name: IBM 2964 N96 400 (z/VM 6.4.0) > Krnl PSW : 0404e00180000000 0000000026c2b96e > (__free_pages_ok+0x34e/0x5d8) > R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3 > Krnl GPRS: 0000000088d43af7 0000000000484000 000000000000007c > 000000000000000f > 000003d080012100 000003d080013fc0 0000000000000000 > 0000000000100000 > 00000000275cca48 0000000000000100 0000000000000008 > 000003d080010000 > 00000000000001d0 000003d000000000 0000000026c2b78a > 000000002717fdb0 > Krnl Code: 0000000026c2b95c: ec1100b30659 risbgn %r1,%r1,0,179,6 > 0000000026c2b962: e32014000036 pfd 2,1024(%r1) > #0000000026c2b968: d7ff10001000 xc 0(256,%r1),0(%r1) > >0000000026c2b96e: 41101100 la %r1,256(%r1) > 0000000026c2b972: a737fff8 brctg %r3,26c2b962 > 0000000026c2b976: d7ff10001000 xc 0(256,%r1),0(%r1) > 0000000026c2b97c: e31003400004 lg %r1,832 > 0000000026c2b982: ebff1430016a asi 5168(%r1),-1 > Call Trace: > __free_pages_ok+0x16a/0x5d8) > memblock_free_all+0x206/0x290 > mem_init+0x58/0x120 > start_kernel+0x2b0/0x570 > startup_continue+0x6a/0xc0 > INFO: lockdep is turned off. > Last Breaking-Event-Address: > __free_pages_ok+0x372/0x5d8 > Kernel panic - not syncing: Fatal exception: panic_on_oops > 00: HCPGIR450W CP entered; disabled wait PSW 00020001 80000000 00000000 > 26A2379C > > ... > > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1175,11 +1175,11 @@ static __always_inline bool free_pages_prepare(struct page *page, > debug_check_no_obj_freed(page_address(page), > PAGE_SIZE << order); > } > - arch_free_page(page, order); > if (want_init_on_free()) > kernel_init_free_pages(page, 1 << order); > > kernel_poison_pages(page, 1 << order, 0); > + arch_free_page(page, order); > if (debug_pagealloc_enabled()) > kernel_map_pages(page, 1 << order, 0); AFAICT the meticulously undocumented s390 set_page_unused() will cause there to be a fault if anyone tries to access the page contents, yes? So I think you've moved the arch_free_page() to be after the final thing which can access page contents, yes? If so, we should have a comment in free_pages_prepare() to attmept to prevent this problem from reoccurring as the code evolves?
