Re: [Fastboot] [PATCH] kexec: Avoid overwriting the current pgd (i386)

["Magnus Damm" <magnus.damm@gmail.com>]

On 5/1/06, Vivek Goyal <vgoyal@in.ibm.com> wrote:
> On Mon, May 01, 2006 at 06:49:16PM +0900, Magnus Damm wrote:
> > kexec: Avoid overwriting the current pgd (i386)
> >
> > This patch upgrades the i386-specific kexec code to avoid overwriting the
> > current pgd. Overwriting the current pgd is bad when CONFIG_CRASH_DUMP is used
> > to start a secondary kernel that dumps the memory of the previous kernel.
> >
> > The code introduces a new set of page tables called "page_table_a". These
> > tables are used to provide an executable identity mapping without overwriting
> > the current pgd.
>
> True, current pgd is overwritten but that effects only user space mappings
> and currently "crash" supports only backtracing kernel space code. But at
> the same time probably it is not a bad idea to maintain a separate page
> table and switch to that instead of overwriting the existing pgd. This
> shall help if in future user space backtracing is also supported.

I agree, but I also think that overwriting user space mappings is bad
from the trace perspective too, especially if you look at kernel data.
I would say that user space page tables are just another kernel data
structure, and overwriting them may result in things like inconsistent
page->mapcount values.

Thanks for the comments,

/ magnus