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SubjectRe: [Fastboot] [PATCH] Reserving backup region for kexec based crashdumps.
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Hirokazu Takahashi <taka@valinux.co.jp> writes:

> Hi Vivek,
>
> > > Hi Vivek and Eric,
> > >
> > > IMHO, why don't we swap not only the contents of the top 640K
> > > but also kernel working memory for kdump kernel?
> >
> >
> > Initial patches of kdump had adopted the same approach but given the
> > fact devices are not stopped during transition to new kernel after a
> > panic, it carried inherent risk of some DMA going on and corrupting the
> > new kernel/data structures. Hence the idea of running the kernel from a
> > reserved location came up. This should be DMA safe as long as DMA is not
> > misdirected.
>
> I see, that makes sense.
> But I'm not sure yet that it's safe to access the top of 640MB.
640K?

> I wonder how kmalloc(GFP_DMA) works in a kdump kernel.

All that happens there is a one line change to vmlinux.lds.S that
causes the kernel to live at a different physical and virtual
address. So everything works as normal.

I do agree that it is risky to use the first 640K for normal work.
But on the list of things to fix it is a minor war, and even if we
back up that region of memory we don't need to use it.

There are still remain a lot of code reviews to ensure the code is
generally safe.

Eric
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