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

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Date:	Tue, 19 Mar 2013 14:22:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>, vgoyal@...hat.com,
	cpw@....com, kumagai-atsushi@....nes.nec.co.jp,
	lisa.mitchell@...com, heiko.carstens@...ibm.com,
	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

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Sat, 16 Mar 2013 13:02:29 +0900 HATAYAMA Daisuke <d.hatayama@...fujitsu.com> wrote:
>
>> 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.
>
> I don't really understand this.  Why does the number of or size of
> note segments affect their alignment?
>
>> --- a/fs/proc/vmcore.c
>> +++ b/fs/proc/vmcore.c
>> @@ -38,6 +38,8 @@ static u64 vmcore_size;
>>  
>>  static struct proc_dir_entry *proc_vmcore = NULL;
>>  
>> +static bool support_mmap_vmcore;
>
> This is quite regrettable.  It means that on some kernels/machines,
> mmap(vmcore) simply won't work.  This means that people might write
> code which works for them, but which will fail for others when deployed
> on a small number of machines.
>
> Can we avoid this?  Why can't we just copy the notes even if there are
> a large number of them?

Yes.  If it simplifies things I don't see a need to support mmapping
everything.  But even there I don't see much of an issue.

Today we allocate a buffer to hold the ELF header program headers and
the note segment, and we could easily allocate that buffer in such a way
to make it mmapable.

Eric
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