linux-kernel - Re: [PATCH v22 5/9] arm64: kdump: Reimplement crashkernel=X

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <YmgzxsrrMlCDYsWp@arm.com>
Date:   Tue, 26 Apr 2022 19:02:46 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, Dave Young <dyoung@...hat.com>,
        Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Chen Zhou <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>
Subject: Re: [PATCH v22 5/9] arm64: kdump: Reimplement crashkernel=X

On Thu, Apr 14, 2022 at 07:57:16PM +0800, Zhen Lei wrote:
>  /*
>   * reserve_crashkernel() - reserves memory for crash kernel
>   *
>   * This function reserves memory area given in "crashkernel=" kernel command
>   * line parameter. The memory reserved is used by dump capture kernel when
>   * primary kernel is crashing.
> + *
> + * NOTE: Reservation of crashkernel,low is special since its existence
> + * is not independent, need rely on the existence of crashkernel,high.
> + * Here, four cases of crashkernel low memory reservation are summarized:
> + * 1) crashkernel=Y,low is specified explicitly, the size of crashkernel low
> + *    memory takes Y;
> + * 2) crashkernel=,low is not given, while crashkernel=,high is specified,
> + *    take the default crashkernel low memory size;
> + * 3) crashkernel=X is specified, while fallback to get a memory region
> + *    in high memory, take the default crashkernel low memory size;
> + * 4) crashkernel='invalid value',low is specified, failed the whole
> + *    crashkernel reservation and bail out.

Following the x86 behaviour made sense when we were tried to get that
code generic. Now that we moved the logic under arch/arm64, we can
diverge a bit. I lost track of the original (v1/v2) proposal but I
wonder whether we still need the fallback to high for crashkernel=Y.
Maybe simpler, no fallbacks:

	crashkernel=Y - keep the current behaviour, ignore high,low
	crashkernel=Y,high - allocate above ZONE_DMA
	crashkernel=Y,low - allocate within ZONE_DMA

>From your proposal, the difference is that the Y,high option won't
have any default ZONE_DMA fallback, one would have to explicitly pass
the Y,low option if needed.

Just a thought, maybe it makes the code simpler. But I'm open to
discussion if there are good arguments for the proposed (x86-like)
behaviour. One argument could be for crashkernel=Y to fall back to high
if distros don't want to bother with high/low settings.

Another thing I may have asked in the past, what happens if we run a new
kernel with these patches with old kexec user tools. I suspect the
crashkernel=Y with the fallback to high will confuse the tools.

BTW, please separate the NO_BLOCK_MAPPINGS optimisations from the
crashkernel above 4G. Let's get the crashkernel reservations sorted
first, it's been around for too long.

Thanks.

-- 
Catalin