linux-kernel - Re: [PATCH 2/6] kexec: delete a useless check in crash_shrink

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Message-ID: <ZHaSF4EGNwSGhGAL@MiWiFi-R3L-srv>
Date:   Wed, 31 May 2023 08:17:51 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/6] kexec: delete a useless check in
 crash_shrink_memory()

On 05/27/23 at 08:34pm, Zhen Lei wrote:
> The check '(crashk_res.parent != NULL)' is added by
> commit e05bd3367bd3 ("kexec: fix Oops in crash_shrink_memory()"), but it's
> stale now. Because if 'crashk_res' is not reserved, it will be zero in
> size and will be intercepted by the above 'if (new_size >= old_size)'.
> 
> Ago:
> 	if (new_size >= end - start + 1)
> 
> Now:
> 	old_size = (end == 0) ? 0 : end - start + 1;
> 	if (new_size >= old_size)

Hmm, I would strongly suggest we keep that check. Even though the
current code like above can do the acutal checking, but its actual usage
is not obvious for checking of crashk_res existence. In the future,
someone may change above calculation and don't notice the hidden
functionality at all behind the calculation. The cost of the check is
almost zero, right?

> 
> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  kernel/kexec_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 22acee18195a591..d1ab139dd49035e 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -1137,7 +1137,7 @@ int crash_shrink_memory(unsigned long new_size)
>  	end = start + new_size;
>  	crash_free_reserved_phys_range(end, crashk_res.end);
>  
> -	if ((start == end) && (crashk_res.parent != NULL))
> +	if (start == end)
>  		release_resource(&crashk_res);
>  
>  	ram_res->start = end;
> -- 
> 2.25.1
>