On Mon, Sep 28, 2015 at 04:07:57PM +0800, Baoquan He wrote:
> On 09/28/15 at 02:41pm, Lee, Chun-Yi wrote:
> > On big machines have CPU number that's very nearly to consume whole ELF
> > headers buffer that's page aligned, 4096, 8192... Then the page fault error
> > randomly happened.
> >
> > This patch modified the code in fill_up_crash_elf_data() by using
> > walk_system_ram_res() instead of walk_system_ram_range() to count the max
> > number of crash memory ranges. That's because the walk_system_ram_range()
> > filters out small memory regions that reside the same page, but
> > walk_system_ram_res() does not.
> >
> > The oringial page fault issue sometimes happened on big machines when
> > preparing ELF headers:
> >
> > [ 305.291522] BUG: unable to handle kernel paging request at
> > ffffc90613fc9000
> > [ 305.299621] IP: [<ffffffff8103d645>]
> > prepare_elf64_ram_headers_callback+0x165/0x260
> > [ 305.308300] PGD e000032067 PUD 6dcbec54067 PMD 9dc9bdeb067 PTE 0
> > [ 305.315393] Oops: 0002 [#1] SMP
> > [...snip]
> > [ 305.420953] task: ffff8e1c01ced600 ti: ffff8e1c03ec2000 task.ti:
> > ffff8e1c03ec2000
> > [ 305.429292] RIP: 0010:[<ffffffff8103d645>] [<ffffffff8103d645>]
> > prepare_elf64_ra
> > m_headers_callback+0x165/0x260
> > [...snip]
> >
> > After tracing prepare_elf64_headers() and
> > prepare_elf64_ram_headers_callback(),
> > the code uses walk_system_ram_res() to fill-in crash memory regions
> > information
> > to program header, so it counts those small memory regions that reside in a
> > page area. But, when kernel was using walk_system_ram_range() in
> > fill_up_crash_elf_data() to count the number of crash memory regions, it
> > filters out small regions.
> >
> > I printed those small memory regions, for example:
> >
> > kexec: Get nr_ram ranges. vaddr=0xffff880077592258 paddr=0x77592258,
> > sz=0xdc0
> >
> > Base on the logic of walk_system_ram_range(), this memory region will be
> > filter out:
> >
> > pfn = (0x77592258 + 0x1000 - 1) >> 12 = 0x77593
> > end_pfn = (0x77592258 + 0xfc0 -1 + 1) >> 12 = 0x77593
> > end_pfn - pfn = 0x77593 - 0x77593 = 0 <=== if (end_pfn > pfn) [FAIL]
> >
> > So, the max_nr_ranges that counted by kernel doesn't include small memory
> > regions. That causes the page fault issue happened in later code path for
> > preparing EFL headers,
> >
> > This issue was hided on small machine that doesn't have too many CPU because
> > the free space of ELF headers buffer can cover the number of small memory
> > regions. But, when the machine has more CPUs or the number of memory regions
> > very nearly to consume whole page aligned buffer, e.g. 4096, 8192... Then
> > issue will happen randomly.
>
> CC akpm too.
>
> Read code again and I think it makes sense to use walk_system_ram_res.
> And in prepare_elf64_headers it also uses walk_system_ram_res. That's
> why you can find this bug. Otherwise we never find this and those small
> regions which only spread in one page will be lost in vmcore.
>
> Besides could you please rearrange your patch log? It's not easy to get
> what this patch have done.
>
To avoid confusing, I will simplify the patch description.
Removing things about CPU number but keep the difference between
walk_system_ram_res and walk_system_ram_range.
Thanks a lot!
Joey Lee
> >
> > Signed-off-by: Lee, Chun-Yi <j...@suse.com>
> > ---
> > arch/x86/kernel/crash.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> > index e068d66..ad273b3d 100644
> > --- a/arch/x86/kernel/crash.c
> > +++ b/arch/x86/kernel/crash.c
> > @@ -185,8 +185,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
> > }
> >
> > #ifdef CONFIG_KEXEC_FILE
> > -static int get_nr_ram_ranges_callback(unsigned long start_pfn,
> > - unsigned long nr_pfn, void *arg)
> > +static int get_nr_ram_ranges_callback(u64 start, u64 end, void *arg)
> > {
> > int *nr_ranges = arg;
> >
> > @@ -214,7 +213,7 @@ static void fill_up_crash_elf_data(struct
> > crash_elf_data *ced,
> >
> > ced->image = image;
> >
> > - walk_system_ram_range(0, -1, &nr_ranges,
> > + walk_system_ram_res(0, -1, &nr_ranges,
> > get_nr_ram_ranges_callback);
> >
> > ced->max_nr_ranges = nr_ranges;
> > --
> > 2.1.4
> >
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