linux-kernel - Re: [RFC/RFT PATCH 1/5] s390: make crashk

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Date:   Tue, 1 Jun 2021 15:18:36 +0200
From:   Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org
Subject: Re: [RFC/RFT PATCH 1/5] s390: make crashk_res resource a child of
 "System RAM"

On Mon, 31 May 2021 15:29:55 +0300
Mike Rapoport <rppt@...nel.org> wrote:

> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> Commit 4e042af463f8 ("s390/kexec: fix crash on resize of reserved memory")
> added a comment that says "crash kernel resource should not be part of the
> System RAM resource" but never explained why. As it looks from the code in
> the kernel and in kexec there is no actual reason for that.

Still testing, but so far everything works fine.

> 
> Keeping crashk_res inline with other resources makes code simpler and
> cleaner, and allows future consolidation of the resources setup across
> several architectures.
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>  arch/s390/kernel/setup.c | 21 +++++----------------
>  1 file changed, 5 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
> index 5aab59ad5688..30430e7c1b03 100644
> --- a/arch/s390/kernel/setup.c
> +++ b/arch/s390/kernel/setup.c
> @@ -500,6 +500,9 @@ static struct resource __initdata *standard_resources[] = {
>  	&code_resource,
>  	&data_resource,
>  	&bss_resource,
> +#ifdef CONFIG_CRASH_DUMP
> +	&crashk_res,
> +#endif
>  };
>  
>  static void __init setup_resources(void)
> @@ -535,7 +538,7 @@ static void __init setup_resources(void)
>  
>  		for (j = 0; j < ARRAY_SIZE(standard_resources); j++) {
>  			std_res = standard_resources[j];
> -			if (std_res->start < res->start ||
> +			if (!std_res->end || std_res->start < res->start ||
>  			    std_res->start > res->end)
>  				continue;
>  			if (std_res->end > res->end) {

Why is this extra check for !std_res->end added here? I assume it
might be needed later, after you moved this to common code, but I
cannot see how any of the other patches in this series would require
that.

> @@ -552,20 +555,6 @@ static void __init setup_resources(void)
>  			}
>  		}
>  	}
> -#ifdef CONFIG_CRASH_DUMP
> -	/*
> -	 * Re-add removed crash kernel memory as reserved memory. This makes
> -	 * sure it will be mapped with the identity mapping and struct pages
> -	 * will be created, so it can be resized later on.
> -	 * However add it later since the crash kernel resource should not be
> -	 * part of the System RAM resource.
> -	 */
> -	if (crashk_res.end) {
> -		memblock_add_node(crashk_res.start, resource_size(&crashk_res), 0);
> -		memblock_reserve(crashk_res.start, resource_size(&crashk_res));
> -		insert_resource(&iomem_resource, &crashk_res);
> -	}
> -#endif
>  }
>  
>  static void __init setup_ident_map_size(void)
> @@ -733,7 +722,7 @@ static void __init reserve_crashkernel(void)
>  		diag10_range(PFN_DOWN(crash_base), PFN_DOWN(crash_size));
>  	crashk_res.start = crash_base;
>  	crashk_res.end = crash_base + crash_size - 1;
> -	memblock_remove(crash_base, crash_size);
> +	memblock_reserve(crash_base, crash_size);
>  	pr_info("Reserving %lluMB of memory at %lluMB "
>  		"for crashkernel (System RAM: %luMB)\n",
>  		crash_size >> 20, crash_base >> 20,

Other architectures check the return value of memblock_reserve() at
this point, and exit crashkernel reservation if it fails. IIUC, the
only reason why memblock_reserve() could fail would be the same reason
why also memblock_remove() could fail, i.e. that memblock_double_array()
would fail. And since we also do not check that at the moment, your
patch would probably not (additionally) break anything.

Still, this might be something for an add-on patch (for us). Do you
happen to know how likely it would be that memblock_remove/reserve()
could fail at this point?