linux-kernel - Re: [PATCH v13 03/16] s390, kexec_file: drop arch_kexec_mem

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Date:   Wed, 1 Aug 2018 10:29:51 +0200
From:   Philipp Rudo <prudo@...ux.ibm.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     catalin.marinas@....com, will.deacon@....com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, dyoung@...hat.com, bhe@...hat.com,
        arnd@...db.de, schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        ard.biesheuvel@...aro.org, james.morse@....com,
        bhsharma@...hat.com, kexec@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v13 03/16] s390, kexec_file: drop arch_kexec_mem_walk()

Hey Akashi,

I kept thinking about this patch and remembered why I didn't made the change
you are suggesting now.

The problem is when you only check for kbuf->mem you are excluding address 0,
which might be a valid address to load the kernel to. On s390 this is actually
done when the kernel is not loaded via a boot loader. For kexec_file however,
we cut off the first few kB of the image and jump directly to 'startup'. So
checking for !0 does not cause a problem here.

Anyway, the long term safer solution would be something like

#define KEXEC_BUF_MEM_UNKNOWN (-1UL)

for architectures to tell common code to search a fitting mem hole.

Back then I didn't do the change because I had the other workaround, which
didn't require a common code change. But when you are touching the code now it
is worth thinking about it.

Just wanted to let you know
Philipp


On Wed,  1 Aug 2018 16:58:07 +0900
AKASHI Takahiro <takahiro.akashi@...aro.org> wrote:

> Since s390 already knows where to locate buffers, calling
> arch_kexec_mem_walk() has no sense. So we can just drop it as kbuf->mem
> indicates this while all other architectures sets it to 0 initially.
> 
> This change is a preparatory work for the next patch, where all the
> variant memory walks, either on system resource or memblock, will be
> put in one common place so that it will satisfy all the architectures'
> need.
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> Reviewed-by: Philipp Rudo <prudo@...ux.ibm.com>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: Dave Young <dyoung@...hat.com>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: Baoquan He <bhe@...hat.com>
> ---
>  arch/s390/kernel/machine_kexec_file.c | 10 ----------
>  kernel/kexec_file.c                   |  4 ++++
>  2 files changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
> index f413f57f8d20..32023b4f9dc0 100644
> --- a/arch/s390/kernel/machine_kexec_file.c
> +++ b/arch/s390/kernel/machine_kexec_file.c
> @@ -134,16 +134,6 @@ int kexec_file_add_initrd(struct kimage *image, struct s390_load_data *data,
>  	return ret;
>  }
>  
> -/*
> - * The kernel is loaded to a fixed location. Turn off kexec_locate_mem_hole
> - * and provide kbuf->mem by hand.
> - */
> -int arch_kexec_walk_mem(struct kexec_buf *kbuf,
> -			int (*func)(struct resource *, void *))
> -{
> -	return 1;
> -}
> -
>  int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
>  				     Elf_Shdr *section,
>  				     const Elf_Shdr *relsec,
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 63c7ce1c0c3e..bf39df5e5bb9 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -534,6 +534,10 @@ int kexec_locate_mem_hole(struct kexec_buf *kbuf)
>  {
>  	int ret;
>  
> +	/* Arch knows where to place */
> +	if (kbuf->mem)
> +		return 0;
> +
>  	ret = arch_kexec_walk_mem(kbuf, locate_mem_hole_callback);
>  
>  	return ret == 1 ? 0 : -EADDRNOTAVAIL;