Date:   Wed, 8 Mar 2023 12:02:02 +0100
From:   Simon Horman <horms@kernel.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, catalin.marinas@arm.com,
        thunder.leizhen@huawei.com, John.p.donnelly@oracle.com,
        will@kernel.org, kexec@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4] arm64: kdump: simplify the reservation behaviour of
 crashkernel=,high
Message-ID: <ZAhrKkld4oo2EhVz@kernel.org>
References: <20230306084124.300584-1-bhe@redhat.com>
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On Mon, Mar 06, 2023 at 04:41:24PM +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.
> 
> While we observed an unexpected case where a reserved region crosses the
> high and low meomry boundary. E.g on a system with 4G as low memory end,
> user added the kernel parameters like: 'crashkernel=512M,high', it could
> finally have [4G-126M, 4G+386M], [1G, 1G+128M] regions in running kernel.
> The crashkernel high region crossing low and high memory boudary will bring
> issues:
> 
> 1) For crashkernel=x,high, if getting crashkernel high region across
> low and high memory boundary, then user will see two memory regions in
> low memory, and one memory region in high memory. The two crashkernel
> low memory regions are confusing as shown in above example.
> 
> 2) If people explicityly specify "crashkernel=x,high crashkernel=y,low"
> and y <= 128M, when crashkernel high region crosses low and high memory
> boundary and the part of crashkernel high reservation below boundary is
> bigger than y, the expected crahskernel low reservation will be skipped.
> But the expected crashkernel high reservation is shrank and could not
> satisfy user space requirement.
> 
> 3) The crossing boundary behaviour of crahskernel high reservation is
> different than x86 arch. On x86_64, the low memory end is 4G fixedly,
> and the memory near 4G is reserved by system, e.g for mapping firmware,
> pci mapping, so the crashkernel reservation crossing boundary never happens.
> >From distros point of view, this brings inconsistency and confusion. Users
> need to dig into x86 and arm64 system details to find out why.
> 
> For kernel itself, the impact of issue 3) could be slight. While issue
> 1) and 2) cause actual impact because it brings obscure semantics and
> behaviour to crashkernel=,high reservation.
> 
> Here, for crashkernel=xM,high, search the high memory for the suitable
> region only in high memory. If failed, try reserving the suitable
> region only in low memory. Like this, the crashkernel high region will
> only exist in high memory, and crashkernel low region only exists in low
> memory. The reservation behaviour for crashkernel=,high is clearer and
> simpler.
> 
> Note: On arm64, the high and low memory boudary could be 1G if it's RPi4
> system, or 4G if other normal systems.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>

Reviewed-by: Simon Horman <horms@kernel.org>