linux-kernel - Re: [PATCH v5 1/5] vmcore: Introduce ELF header in new memory feature

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Date:	Mon, 1 Jul 2013 13:37:27 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Jan Willeke <willeke@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH v5 1/5] vmcore: Introduce ELF header in new memory feature

On Fri, Jun 28, 2013 at 10:15:52AM +0200, Michael Holzheu wrote:
> On Thu, 27 Jun 2013 16:23:34 -0400
> Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > On Thu, Jun 27, 2013 at 03:32:02PM -0400, Vivek Goyal wrote:
> > > On Fri, Jun 21, 2013 at 04:17:03PM +0200, Michael Holzheu wrote:
> > > > On Fri, 14 Jun 2013 14:54:02 -0400
> > > > Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> [snip]
> 
> > Thinking more about it, I think let us cleanup with this little ugly
> > bit too so that future changes become easy.
> > 
> > Current convention is that elfcorehdr_addr and elfcorehdr_size are
> > already set by arch code by the time vmcore.c starts reading it. Can't
> > s390 allocate elf headers in early boot code and elfcorehdr_addr? Then
> > we don't have to call elfcorehdr_alloc().
> > 
> > And once we are done with reading headers, we can call elfcorehdr_free()
> > and s390 could free memory and set elfcorehdr_addr to ELFCORE_ADDR_ERR
> > and elfcorehdr_size=0. That would signify that one can not try to read
> > elf headers now and it must have been freed.
> > 
> > is_kdump_kernel() will continue to work as elfcorehdr_addr is
> > ELFCORE_ADDR_ERR. And that will mean that either elfcorehdr were not
> > readable/usable to begin with or they have been freed now.
> 
> Hello Vivek,
> 
> We would like to keep the alloc/free symmetry as you have suggested in a
> previous mail. This also has the advantage that we do not have to rely
> on the ordering of init calls.
> 
> Wouldn't it be sufficient to just set elfcorehdr_addr to ELFCORE_ADDR_ERR
> after elfcorehdr_free() and remove the comment?
> 

Hi Michael,

This has only one problem and that is what's the initialization semantics
of elfcorehdr_addr. 

So far we expected it to be initialized very early in boot process and
once this is set, any component in the system could figure out if it
is kdump kernel (is_kdump_kernel()) and do kdump specific things.

But if we move initialization of this variable little late, then it
might be a problem for early users of is_kdump_kernel(). Though right
now I don't see drivers making use of it and only arch specific early
boot up code seems to have it.

So either we can stick to existing semantics of initializing headers
early or we could create a separate variable for is_kdump_kernel() which
is set in early boot and then one can delay initialization of
elfcorehdr_addr() in vmcore_init().

Given the fact that I don't see any users of is_kdump_kernel() in arch
independent directory, and I am assuming that you will tackle all early
users of is_kdump_kernel() in s390, I will be fine even with your patch
below.

Thanks
Vivek


> So the code would look like the following:
> 
> static int __init vmcore_init(void)
> {
>         int rc = 0;
> 
>         /* Allow architectures to allocate ELF header in 2nd kernel */
>         rc = elfcorehdr_alloc(&elfcorehdr_addr, &elfcorehdr_size);
>         if (rc)
>                 return rc;
>         /*
>          * If elfcorehdr= has been passed in cmdline or created in 2nd kernel,
>          * then capture the dump.
>          */
>         if (!(is_vmcore_usable()))
>                 return rc;

>         rc = parse_crash_elf_headers();
>         if (rc) {
>                 pr_warn("Kdump: vmcore not initialized\n");
>                 return rc;
>         }
>         elfcorehdr_free(elfcorehdr_addr);
> 	elfcorehdr_addr = ELFCORE_ADDR_ERR;
> 
>         proc_vmcore = proc_create("vmcore", S_IRUSR, NULL, &proc_vmcore_operations);
>         if (proc_vmcore)
>                 proc_vmcore->size = vmcore_size;
>         return 0;
> }
> 
> This looks clean for me.  > 
> What do you think?
> 
> Best Regards,
> Michael
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