Hello, We have a robot which runs criu tests on linux-next kernels. All tests passed on 4.17.0-rc3-next-20180502. But the 4.17.0-rc3-next-20180504 kernel didn't boot. git bisect points on this patch. On Thu, Apr 26, 2018 at 04:26:19PM -0400, Pavel Tatashin wrote: > The following two bugs were reported by Fengguang Wu: > > kernel reboot-without-warning in early-boot stage, last printk: > early console in setup code > > http://lkml.kernel.org/r/20180418135300.inazvpxjxowogyge@xxxxxxxxxxxxxxxxxxxxxx The problem looks similar with this one. [ 5.596975] devtmpfs: mounted [ 5.855754] Freeing unused kernel memory: 1704K [ 5.858162] Write protecting the kernel read-only data: 18432k [ 5.860772] Freeing unused kernel memory: 2012K [ 5.861838] Freeing unused kernel memory: 160K [ 5.862572] rodata_test: all tests were successful [ 5.866857] random: fast init done early console in setup code [ 0.000000] Linux version 4.17.0-rc3-00023-g7c4cc2d022a1 (avagin@laptop) (gcc version 8.0.1 20180324 (Red Hat 8.0.1-0.20) (GCC)) #13 SMP Fri May 4 01:10:51 PDT 2018 [ 0.000000] Command line: root=/dev/vda2 ro debug console=ttyS0,115200 LANG=en_US.UTF-8 slub_debug=FZP raid=noautodetect selinux=0 earlyprintk=serial,ttyS0,115200 [ 0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers' [ 0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers' [ 0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers' [ 0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers' $ git describe HEAD v4.17-rc3-23-g7c4cc2d022a1 [avagin@laptop linux-next]$ git log --pretty=oneline | head -n 1 7c4cc2d022a1fd56eb2ee555533b8666bc780f1e mm: access to uninitialized struct page > > And, also: > [per_cpu_ptr_to_phys] PANIC: early exception 0x0d > IP 10:ffffffffa892f15f error 0 cr2 0xffff88001fbff000 > > http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@xxxxxxxxxxxxxxxxxxxxxx > > Both of the problems are due to accessing uninitialized struct page from > trap_init(). We must first do mm_init() in order to initialize allocated > struct pages, and than we can access fields of any struct page that belongs > to memory that's been allocated. > > Below is explanation of the root cause. > > The issue arises in this stack: > > start_kernel() > trap_init() > setup_cpu_entry_areas() > setup_cpu_entry_area(cpu) > get_cpu_gdt_paddr(cpu) > per_cpu_ptr_to_phys(addr) > pcpu_addr_to_page(addr) > virt_to_page(addr) > pfn_to_page(__pa(addr) >> PAGE_SHIFT) > The returned "struct page" is sometimes uninitialized, and thus > failing later when used. It turns out sometimes is because it depends > on KASLR. > > When boot is failing we have this when pfn_to_page() is called: > kasrl: 0x000000000d600000 > addr: ffffffff83e0d000 > pa: 1040d000 > pfn: 1040d > page: ffff88001f113340 > page->flags ffffffffffffffff <- Uninitialized! > > When boot is successful: > kaslr: 0x000000000a800000 > addr: ffffffff83e0d000 > pa: d60d000 > pfn: d60d > page: ffff88001f05b340 > page->flags 280000000000 <- Initialized! > > Here are physical addresses that BIOS provided to us: > e820: BIOS-provided physical RAM map: > BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable > BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved > BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved > BIOS-e820: [mem 0x0000000000100000-0x000000001ffdffff] usable > BIOS-e820: [mem 0x000000001ffe0000-0x000000001fffffff] reserved > BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved > BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved > > In both cases, working and non-working the real physical address is > the same: > > pa - kasrl = 0x2E0D000 > > The only thing that is different is PFN. > > We initialize struct pages in four places: > > 1. Early in boot a small set of struct pages is initialized to fill > the first section, and lower zones. > 2. During mm_init() we initialize "struct pages" for all the memory > that is allocated, i.e reserved in memblock. > 3. Using on-demand logic when pages are allocated after mm_init call > 4. After smp_init() when the rest free deferred pages are initialized. > > The above path happens before deferred memory is initialized, and thus > it must be covered either by 1, 2 or 3. > > So, lets check what PFNs are initialized after (1). > > memmap_init_zone() is called for pfn ranges: > 1 - 1000, and 1000 - 1ffe0, but it quits after reaching pfn 0x10000, > as it leaves the rest to be initialized as deferred pages. > > In the working scenario pfn ended up being below 1000, but in the > failing scenario it is above. Hence, we must initialize this page in > (2). But trap_init() is called before mm_init(). > > The bug was introduced by "mm: initialize pages on demand during boot" > because we lowered amount of pages that is initialized in the step > (1). But, it still could happen, because the number of initialized > pages was a guessing. > > The current fix moves trap_init() to be called after mm_init, but as > alternative, we could increase pgdat->static_init_pgcnt: > In free_area_init_node we can increase: > pgdat->static_init_pgcnt = min_t(unsigned long, PAGES_PER_SECTION, > pgdat->node_spanned_pages); > Instead of one PAGES_PER_SECTION, set several, so the text is > covered for all KASLR offsets. But, this would still be guessing. > Therefore, I prefer the current fix. > > Fixes: c9e97a1997fb ("mm: initialize pages on demand during boot") > > Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> > Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> > --- > init/main.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/init/main.c b/init/main.c > index b795aa341a3a..870f75581cea 100644 > --- a/init/main.c > +++ b/init/main.c > @@ -585,8 +585,8 @@ asmlinkage __visible void __init start_kernel(void) > setup_log_buf(0); > vfs_caches_init_early(); > sort_main_extable(); > - trap_init(); > mm_init(); > + trap_init(); > > ftrace_init(); >
