Re: [PATCH v17 03/10] x86: kdump: use macro CRASH_ADDR_LOW_MAX in functions reserve_crashkernel()

Baoquan He
2021-12-16T14:11:50Z

On 12/16/21 at 11:55am, Borislav Petkov wrote:
> On Thu, Dec 16, 2021 at 09:10:40AM +0800, Baoquan He wrote:
> > reserve_crashkernel_low() always return 0 on x86_32, so the not equivalent
> > transformation for x86_32 doesn't matter, I think.
>
> That is, of course, very obvious... not!
>
> Why is that function parsing crashkernel=XM, and crashkernel=X,high,
> then it attempts some low memory reservation too? Why isn't
> crashkernel=Y,low parsed there too?
>
> I guess this alludes to why:
>
> crashkernel=size[KMG],low
> [KNL, X86-64] range under 4G. When crashkernel=X,high
> is passed, kernel could allocate physical memory region
> above 4G, that cause second kernel crash on system
> that require some amount of low memory, e.g. swiotlb
> requires at least 64M+32K low memory, also enough extra
> low memory is needed to make sure DMA buffers for 32-bit
> devices won't run out.
>
> So, before this is going anywhere, I'd like to see this function
> documented properly. I see Documentation/admin-guide/kdump/kdump.rst
> explains the crashkernel= options too so you can refer to it in the
> comments so that when someone looks at that code, someone can follow why
> it is doing what it is doing.
>
> Then, as a future work, all that parsing of crashkernel= cmdline options
> should be concentrated at the beginning and once it is clear what the
> user requests, the reservations should be done.

Totally agree we should refactor code to make reserve_crashkernel()
clearer on logic and readibility. In this patchset, we can rewrite the
kernel-doc of reserve_crashkernel() to add more words to explain. As for
the code refactoring, it can be done in another patchset.

>
> As it is, reserve_crashkernel() is pretty unwieldy and hard to read.