On Fri, 2019-09-27 at 14:02 -0700, Andrew Morton wrote: > 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? 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? Right, something like this above arch_free_page() there? /* * It needs to be just above kernel_map_pages(), as s390 could mark those * pages unused and then trigger a fault when accessing. */
