Re: [PATCH 0/2] kdump/mmap: Fix mmap of /proc/vmcore for s390

From: ebiederm@xmission.com (Eric W. Biederman)
To: Zhang Yanfei <zhangyanfei.yes@gmail.com>
Cc: kexec@lists.infradead.org,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Jan Willeke <willeke@de.ibm.com>,
	linux-kernel@vger.kernel.org,
	HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michael Holzheu <holzheu@linux.vnet.ibm.com>,
	Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [PATCH 0/2] kdump/mmap: Fix mmap of /proc/vmcore for s390
Date: Sat, 25 May 2013 13:36:09 -0700	[thread overview]
Message-ID: <8761y6ajba.fsf@xmission.com> (raw)
In-Reply-To: <51A076FE.3060604@gmail.com> (Zhang Yanfei's message of "Sat, 25 May 2013 16:31:58 +0800")

Zhang Yanfei <zhangyanfei.yes@gmail.com> writes:

> 于 2013年05月25日 11:01, Eric W. Biederman 写道:
>> Zhang Yanfei <zhangyanfei.yes@gmail.com> writes:
>> 
>>> Hello Eric,
>>>
>>>> The function copy_oldmem_page also concerns me.  I don't have a clue why
>>>> we duplicate that function on every architecutre in a slightly different
>>>> form.  There should be enough abstractions in the kernel to make that
>>>> unnecessary.  I would be glad to see that function go, and remove the
>>>> possibility of confusion that happened on s390.
>>>
>>> You mean we should have a common copy_oldmem_page for all architectures? And
>>> just like vivek said above, for s390, we should put the swap info in the elf
>>> headers instead of doing that in copy_oldmem_page.
>> 
>> Exactly.
>> 
>> The user space change in /sbin/kexec should even be backwards compatible
>> for s390.  So fixing /sbin/kexec should probably come first.
>> 
>
> I am kind of not sure about the "backwards compatible for s390" you meant.
>
> For s390, if we put swap info into the elf header, This will change /sbin/kexec.
> But at this point, copy_oldmem_page is still doing the swap when we try to read
> the pages among [0 - OLDMEM_SIZE] and [OLDMEM_BASE - OLDMEM_BASE + OLDMEM_SIZE].
> So removing the swap in copy_oldmem_page should be done at the same time.
> New kexec with old kernels would fail and old kexec with new kernels would fail too.
>
> So could you please explain more about the ""backwards compatible". And please
> correct me if I am wrong.

It looks like my misreading of things.  I was not expecting the existing
copy_oldmem_page to do a complete swap of addresses.  I was expecting
something like the result obtained by doing a header swap with the ELF
headers where part of the address was translated and the rest was simply
not mentioned.

But from other replies it appears there isn't a problem.

Eric

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