Re: /proc/vmcore mmap() failure issue

[Atsushi Kumagai]

On 2013/11/25 23:42:31, kexec <kexec-bounces@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Nov 25, 2013 at 06:01:37PM +0900, HATAYAMA Daisuke wrote:
> 
> [..]
> > > I agree to avoid this issue by fixing makedumpfile as workaround while to
> > > fix kernel is so tough and risky. However, it sounds strange to me to fix
> > > userspace side elaborately for such definite kernel issue whose cause is
> > > known, so we should fix the kernel itself.
> > > 
> > 
> > > Otherwise, will you continue to add specific fixes into user tools to
> > > address kernel issues like this case ?
> > > 
> > 
> > makedumpfile supports a wide range of kernel versions and needs to satisfy
> > backward compatibility. mmap() on /proc/vmcore might be backported to some of
> > the old versions on some distributions if necessary. Then, it's hard to fix
> > each old kernel at each back port. The method that can be applied to all the
> > kernels in general, is necessary.
> > 
> > Also, looking at ia64 case where there's boot loader data on partial pages,
> > there could be other environments where partial pages contain other important
> > data other components have. So, the issue depends not only on kernels but also
> > other components such as boot loader and firmwares that can put data on
> > partial pages. We need to get there as long as there's important data there
> > and we have access to there.
> 
> Hi Atsushi, Hatayama,
> 
> So even if we fix the mmap() issue in kernel, looks like it will be a
> good idea to ship the fix in makedumpfile as there have been a kernel
> release where mmap() will cause issues.

OK, I'll make a patch set for makedumpfile to address issues about mmap():

  1. Fix the partial page issue
  2. Introduce general fall back structure (I already posted)
     http://lists.infradead.org/pipermail/kexec/2013-November/010199.html
  3. Add --non-mmap option


Thanks
Atsushi Kumagai

> Having said that, I think we need to fix it in kernel also. I was not sure
> that what's the right fix. Should we truncate partial pages or should
> we just copy partial pages from old memory to new kernel's memory and fill
> partial page with zeros. And that's why I was hoping that makedumpfile
> can fill the gap.
> 
> Copying partial pages to new memory seems like a safer approach. So may
> be we can take a fix in makeudmpfile and another in kernel.
> 
> Hatayama, I know that in the past your initial mmap() patches were copying
> partial pages to new kernel's memory. Would you like to resurrect that
> patch again?
> 
> Thanks
> Vivek
> 
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