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

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Message-ID: <87txo5bxk4.fsf@xmission.com>
Date:	Wed, 20 Mar 2013 13:55:55 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>, cpw@....com,
	kumagai-atsushi@....nes.nec.co.jp, lisa.mitchell@...com,
	heiko.carstens@...ibm.com, akpm@...ux-foundation.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	zhangyanfei@...fujitsu.com
Subject: Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement

Vivek Goyal <vgoyal@...hat.com> writes:

> On Tue, Mar 19, 2013 at 03:38:45PM -0700, Eric W. Biederman wrote:
>> HATAYAMA Daisuke <d.hatayama@...fujitsu.com> writes:
>> 
>> > If there's some vmcore object that doesn't satisfy page-size boundary
>> > requirement, remap_pfn_range() fails to remap it to user-space.
>> >
>> > Objects that posisbly don't satisfy the requirement are ELF note
>> > segments only. The memory chunks corresponding to PT_LOAD entries are
>> > guaranteed to satisfy page-size boundary requirement by the copy from
>> > old memory to buffer in 2nd kernel done in later patch.
>> >
>> > This patch doesn't copy each note segment into the 2nd kernel since
>> > they amount to so large in total if there are multiple CPUs. For
>> > example, current maximum number of CPUs in x86_64 is 5120, where note
>> > segments exceed 1MB with NT_PRSTATUS only.
>> 
>> So you require the first kernel to reserve an additional 20MB, instead
>> of just 1.6MB.  336 bytes versus 4096 bytes.
>> 
>> That seems like completely the wrong tradeoff in memory consumption,
>> filesize, and backwards compatibility.
>
> Agreed. 
>
> So we already copy ELF headers in second kernel's memory. If we start
> copying notes too, then both headers and notes will support mmap().

The only real is it could be a bit tricky to allocate all of the memory
for the notes section on high cpu count systems in a single allocation.

> For mmap() of memory regions which are not page aligned, we can map
> extra bytes (as you suggested in one of the mails). Given the fact
> that we have one ELF header for every memory range, we can always modify
> the file offset where phdr data is starting to make space for mapping
> of extra bytes.

Agreed ELF file offset % PAGE_SIZE should == physical address % PAGE_SIZE to
make mmap work.

> That way whole of vmcore should be mmappable and user does not have
> to worry about reading part of the file and mmaping the rest.

That sounds simplest.

If core counts on the high end do more than double every 2 years we
might have a problem.  Otherwise making everything mmapable seems easy
and sound.

Eric
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