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Date:   Wed, 26 Dec 2018 09:57:57 +0800
From:   Dave Young <dyoung@redhat.com>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Baoquan He <bhe@redhat.com>, yinghai@kernel.org,
        vgoyal@redhat.com, kexec@lists.infradead.org
Subject: Re: [PATCHv2] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr
Message-ID: <20181226015757.GA12717@dhcp-128-65.nay.redhat.com>
References: <1544760446-506-1-git-send-email-kernelfans@gmail.com>
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On 12/14/18 at 12:07pm, Pingfan Liu wrote:
> Customer reported a bug on a high end server with many pcie devices, where
> kernel bootup with crashkernel=384M, and kaslr is enabled. Even
> though we still see much memory under 896 MB, the finding still failed
> intermittently. Because currently we can only find region under 896 MB,
> if w/0 ',high' specified. Then KASLR breaks 896 MB into several parts
> randomly, and crashkernel reservation need be aligned to 128 MB, that's
> why failure is found. It raises confusion to the end user that sometimes
> crashkernel=X works while sometimes fails.
> If want to make it succeed, customer can change kernel option to
> "crashkernel=384M, high". Just this give "crashkernel=xx@yy" a very
> limited space to behave even though its grammer looks more generic.
> And we can't answer questions raised from customer that confidently:
> 1) why it doesn't succeed to reserve 896 MB;
> 2) what's wrong with memory region under 4G;
> 3) why I have to add ',high', I only require 384 MB, not 3840 MB.
> 
> This patch simplifies the method suggested in the mail [1]. It just goes
> bottom-up to find a candidate region for crashkernel. The bottom-up may be
> better compatible with the old reservation style, i.e. still want to get
> memory region from 896 MB firstly, then [896 MB, 4G], finally above 4G.
> 
> There is one trivial thing about the compatibility with old kexec-tools:
> if the reserved region is above 896M, then old tool will fail to load
> bzImage. But without this patch, the old tool also fail since there is no
> memory below 896M can be reserved for crashkernel.
> 
> [1]: http://lists.infradead.org/pipermail/kexec/2017-October/019571.html
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: yinghai@kernel.org,
> Cc: vgoyal@redhat.com
> Cc: kexec@lists.infradead.org
> 
> ---
> v1->v2:
>   improve commit log
>  arch/x86/kernel/setup.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index d494b9b..60f12c4 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -541,15 +541,18 @@ static void __init reserve_crashkernel(void)
>  
>  	/* 0 means: find the address automatically */
>  	if (crash_base <= 0) {
> +		if (!memblock_bottom_up())
> +			memblock_set_bottom_up(true);

Looking at the memblock_find_in_range_node code, it is allocating
bottom up in case bottom_up is true, but it will try to allocate above
kernel_end:

bottom_up_start = max(start, kernel_end);

If kernel lives very high eg. KASLR case, then this bottom up way does
not help.  So probably previous old version to try 896M first then 4G
then maxmem is better.

>  		/*
>  		 * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
>  		 * as old kexec-tools loads bzImage below that, unless
>  		 * "crashkernel=size[KMG],high" is specified.
>  		 */
>  		crash_base = memblock_find_in_range(CRASH_ALIGN,
> -						    high ? CRASH_ADDR_HIGH_MAX
> -							 : CRASH_ADDR_LOW_MAX,
> -						    crash_size, CRASH_ALIGN);
> +			(max_pfn * PAGE_SIZE), crash_size, CRASH_ALIGN);
> +		if (!memblock_bottom_up())
> +			memblock_set_bottom_up(false);
> +
>  		if (!crash_base) {
>  			pr_info("crashkernel reservation failed - No suitable area found.\n");
>  			return;
> -- 
> 2.7.4
>