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: <20130319130229.fe83c985678146980ecc6102@linux-foundation.org>
Date:	Tue, 19 Mar 2013 13:02:29 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
Cc:	vgoyal@...hat.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 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?

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