linux-kernel - Re: [PATCH v5] arm64: kdump: simplify the reservation behaviour of crashkernel=,high

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Date:   Tue, 16 May 2023 11:00:10 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, catalin.marinas@....com,
        horms@...nel.org, thunder.leizhen@...wei.com,
        John.p.donnelly@...cle.com, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5] arm64: kdump: simplify the reservation behaviour of
 crashkernel=,high

Hi Will,

On 04/14/23 at 03:34pm, Will Deacon wrote:
> Hi,
> 
> On Fri, Apr 07, 2023 at 10:24:19AM +0800, Baoquan He wrote:
> > On arm64, reservation for 'crashkernel=xM,high' is taken by searching for
> > suitable memory region top down. If the 'xM' of crashkernel high memory
> > is reserved from high memory successfully, it will try to reserve
> > crashkernel low memory later accoringly. Otherwise, it will try to search
> > low memory area for the 'xM' suitable region. Please see the details in
> > Documentation/admin-guide/kernel-parameters.txt.
> 
> [...]
> 
> >  arch/arm64/mm/init.c | 44 ++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 34 insertions(+), 10 deletions(-)
> 
> I tried to apply this, but smatch is unhappy with the result:
> 
>   | arch/arm64/mm/init.c:153 reserve_crashkernel() error: uninitialized symbol 'search_base'.
> 
> I _think_ this is a false positive, but I must say that the control flow
> in reserve_crashkernel() is extremely hard to follow so I couldn't be
> sure. If the static checker is struggling, then so will humans!
> 
> Ideally, this would all be restructured to make it easier to follow,
> but in the short term we need something to squash the warning.

I tried to refactor the code as you suggested, while it seems not easy
to do. The complexity comes from several cases which need be handled.
I try my best to write a document with the things I think important to
help understand the code. Please help check if it helps or we just having
the current code is fine.

Thanks
Baoquan