Re: [PATCH] Put each per-cpu kdump ELF notes into a single page

[Petr Tesarik <ptesarik@suse.cz>]

On Thu, 11 Sep 2014 16:01:10 -0400
Vivek Goyal <vgoyal@redhat.com> wrote:

> On Fri, Sep 05, 2014 at 06:33:14PM +0200, Petr Tesarik wrote:
> > On architectures that use percpu-vm, the percpu region is not guaranteed
> > to be contiguous in physical space.
> 
> Petr,
> 
> Which are those arches?

All except nommu. Actually, percpu-km will be used instead even on MMU
if SMP is disabled, but since SMP is pretty standard now, I guess the
vast majority of all kernels out there is affected. ;-)

> > However, fs/proc/vmcore.c expects
> > all ELF notes to be contiguous. If the ELF note happens to occupy
> > two non-adjacent physical pages, part of the note may be read from an
> > incorrect memory location by the kdump kernel, resulting in failure to
> > initialize /proc/vmcore (if the content of the following physical page,
> > incorrectly interpreted as an ELF note specifies a large number), wrong
> > register values or other apparent random memory corruption.
> > 
> > There is currently no mechanism to pass the virtual-to-physical mapping
> > of the percpu allocation to the kdump kernel. So, instead, I'm changing
> > the alignment of the ELF note buffer. Since sizeof(note_buf_t) is less
> > than PAGE_SIZE, aligning the buffer to the nearest higher power of 2
> > is enough to make sure that the buffer cannot cross a page boundary,
> > effectively ensuring that the whole buffer is contiguous in physical
> > space.
> > 
> > Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
> > ---
> >  kernel/kexec.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index 2bee072..cdab59d 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -1610,7 +1610,8 @@ void crash_save_cpu(struct pt_regs *regs, int cpu)
> >  static int __init crash_notes_memory_init(void)
> >  {
> >  	/* Allocate memory for saving cpu registers. */
> > -	crash_notes = alloc_percpu(note_buf_t);
> > +	crash_notes = __alloc_percpu(sizeof(note_buf_t),
> > +				     roundup_pow_of_two(sizeof(note_buf_t)));
> 
> I think some of the changelog should show up here as comment in short
> form. I don't think it is obvious that why we are using __alloc_percpu()
> and why aligning to nearst higher power of 2 is needed here. Please also
> mention here which arches run into issues.

OK, I'll add it as a comment in the code. I'll see if I can make it
short but still understandable.

Thanks,
Petr Tesarik

> Thanks
> Vivek
> 
> >  	if (!crash_notes) {
> >  		pr_warn("Kexec: Memory allocation for saving cpu register states failed\n");
> >  		return -ENOMEM;
> > -- 
> > 1.8.4.5
> > 
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> 
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