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

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Message-ID: <ZHb6IFhwRMkAkNjt@MiWiFi-R3L-srv>
Date:   Wed, 31 May 2023 15:41:20 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Leizhen (ThunderTown)" <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 05/31/23 at 10:19am, Leizhen (ThunderTown) wrote:
> 
> 
> On 2023/5/31 8:17, Baoquan He wrote:
> > 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?
> 
> The cost of the check is negligible. The only downside is that it's hard to
> understand why it's added, and I only found out why by looking at the history
> log. In my opinion, the above 'Now:' is the right fix.

It checks if the resource exists before releasing, just a normal
checking?
> 
> > 
> >>
> >> 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
> >>
> > 
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei
>