Subject: Re: [RFC/RFT PATCH 1/5] s390: make crashk_res resource a child of "System RAM"

From: Mike Rapoport <[email-masked]>
Date: Wed, 2 Jun 2021 09:54:50 +0300

On Tue, Jun 01, 2021 at 03:18:36PM +0200, Gerald Schaefer wrote:
> On Mon, 31 May 2021 15:29:55 +0300
> Mike Rapoport <[email-masked]> wrote:
> 
> > From: Mike Rapoport <[email-masked]>
> > 
> > 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 <[email-masked]>
> > ---
> >  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.

This is needed to avoid requesting empty crash_res.
 
> > @@ -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.

We don't check the return value of memblock_reserve() (or memblock_remove()
for that matter) in vast majority of cases all over the place, but it is
very much unlikely it will fail because there is not enough memory.
 
> 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?

Generally, the worst case scenario is that memblock_remove/reserve() will
need to double one of the memblock arrays. This will require to allocate
several kilobytes of memory, but since the memory this early is mostly free
it should not be a problem.

Looking particularly at s390 case, there are handful of reservations before
setup_resouces(), so we can accommodate more than 100 calls to
memblock_reserve() before we'll need to increase memblock.reserved array.

-- 
Sincerely yours,
Mike.

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