Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement

From: ebiederm@xmission.com (Eric W. Biederman)
To: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: kexec@lists.infradead.org, heiko.carstens@de.ibm.com,
	linux-kernel@vger.kernel.org, lisa.mitchell@hp.com,
	kumagai-atsushi@mxc.nes.nec.co.jp, zhangyanfei@cn.fujitsu.com,
	akpm@linux-foundation.org, cpw@sgi.com, vgoyal@redhat.com
Subject: Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement
Date: Thu, 21 Mar 2013 00:07:12 -0700	[thread overview]
Message-ID: <87a9pxz0wv.fsf@xmission.com> (raw)
In-Reply-To: <20130321.154650.424925595.d.hatayama@jp.fujitsu.com> (HATAYAMA Daisuke's message of "Thu, 21 Mar 2013 15:46:50 +0900 (JST)")

HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> writes:

> From: "Eric W. Biederman" <ebiederm@xmission.com>
> Subject: Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement
> Date: Wed, 20 Mar 2013 23:29:05 -0700
>
>> HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> writes:
>>>
>>> Do you mean for each range represented by each PT_LOAD entry, say:
>>>
>>>   [p_paddr, p_paddr + p_memsz]
>>>
>>> extend it as:
>>>
>>>   [rounddown(p_paddr, PAGE_SIZE), roundup(p_paddr + p_memsz, PAGE_SIZE)].
>>>
>>> not only objects in vmcore_list, but also updating p_paddr and p_memsz
>>> members themselves of each PT_LOAD entry? In other words, there's no
>>> new holes not referenced by any PT_LOAD entry since the regions
>>> referenced by some PT_LOAD entry, themselves are extended.
>> 
>> No.  p_paddr and p_memsz as exported should remain the same.
>> I am suggesting that we change p_offset.
>> 
>> I am suggesting to include the data in the file as if we had changed
>> p_paddr and p_memsz.
>> 
>>> Then, the vmcores seen from read and mmap methods are coincide in the
>>> direction of including both ranges
>>>
>>>   [rounddown(p_paddr, PAGE_SIZE), p_paddr]
>>>
>>> and
>>>
>>>   [p_paddr + p_memsz, roundup(p_paddr + p_memsz, PAGE_SIZE)]
>>>
>>> are included in both vmcores seen from read and mmap methods, although
>>> they are originally not dump target memory, which you are not
>>> problematic for ease of implementation.
>>>
>>> Is there difference here from you understanding?
>> 
>> Preserving the actual PT_LOAD segments p_paddr and p_memsz values is
>> important. p_offset we can change as much as we want.  Which means there
>> can be logical holes in the file between PT_LOAD segments, where we put
>> the extra data needed to keep everything page aligned.
>> 
>
> So, I have to make the same question again. Is it OK if two vmcores
> are different? How do you intend the ``extra data'' to be deal with? I
> mean mmap() has to export part of old memory as the ``extra data''.
>
> If you think OK, I'll fill the ``extra data'' with 0 in case of read
> method. If not OK, I'll fill with the corresponding part of old
> memory.

I think the two having different contents violates the principle of
least surprise.

I think exporting the old memory as the ``extra data'' is the least
surprising and the easiest way to go.

I don't mind filling the extra data with zero's but I don't see the
point.

Eric

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