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

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Date:	Wed, 20 Mar 2013 09:51:33 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	ebiederm@...ssion.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

On Tue, Mar 19, 2013 at 01:02:29PM -0700, Andrew Morton wrote:
> 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?

Actually initially he implemented copying notes to second kernel and I 
suggested to go other way (Tried too hard to save memory in second
kernel). I guess it was not a good idea and copying notes keeps it simple.

Thanks
Vivek
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