linux-kernel - Re: [PATCH v4 7/8] vmcore: treat memory chunks referenced by PT_LOAD program header entries in page-size boundary in vmcore

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Date:	Mon, 29 Apr 2013 15:51:54 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
Cc:	ebiederm@...ssion.com, akpm@...ux-foundation.org, cpw@....com,
	kumagai-atsushi@....nes.nec.co.jp, lisa.mitchell@...com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	zhangyanfei@...fujitsu.com, jingbai.ma@...com
Subject: Re: [PATCH v4 7/8] vmcore: treat memory chunks referenced by PT_LOAD
 program header entries in page-size boundary in vmcore_list

On Sat, Apr 13, 2013 at 09:21:46AM +0900, HATAYAMA Daisuke wrote:
> Treat memory chunks referenced by PT_LOAD program header entries in
> page-size boundary in vmcore_list. Formally, for each range [start,
> end], we set up the corresponding vmcore object in vmcore_list to
> [rounddown(start, PAGE_SIZE), roundup(end, PAGE_SIZE)].
> 
> This change affects layout of /proc/vmcore. The gaps generated by the
> rearrangement are newly made visible to applications as
> holes. Concretely, they are two ranges [rounddown(start, PAGE_SIZE),
> start] and [end, roundup(end, PAGE_SIZE)].

Sorry did not understand this part. So if end is not page aligned, then
we roundup(end, PAGE_SIZE) and increase the PT_LOAD size accordingly?
Similarly for start, we do roundown().

- Can you really rounddown() start? Then you will have to change start
  virtual address in program header and that's not really a good idea.

- So this extra memory for end, we read from old memory and not fill
  with zeros? 

> 
> Signed-off-by: HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
> ---
> 
>  fs/proc/vmcore.c |   26 ++++++++++++++++++++------
>  1 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
> index 029bdc0..cd0f9d9 100644
> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -477,16 +477,23 @@ static int __init process_ptload_program_headers_elf64(char *elfptr,
>  	vmcore_off = elfsz + roundup(phdr_ptr->p_memsz, PAGE_SIZE);
>  
>  	for (i = 0; i < ehdr_ptr->e_phnum; i++, phdr_ptr++) {
> +		u64 paddr, start, end, size;
> +
>  		if (phdr_ptr->p_type != PT_LOAD)
>  			continue;
>  
> +		paddr = phdr_ptr->p_offset;
> +		start = rounddown(paddr, PAGE_SIZE);
> +		end = roundup(paddr + phdr_ptr->p_memsz, PAGE_SIZE);
> +		size = end - start;
> +
>  		/* Add this contiguous chunk of memory to vmcore list.*/
> -		if (vmcore_add(vc_list, phdr_ptr->p_offset, phdr_ptr->p_memsz))
> +		if (vmcore_add(vc_list, start, size))
>  			return -ENOMEM;
>  
>  		/* Update the program header offset. */
> -		phdr_ptr->p_offset = vmcore_off;
> -		vmcore_off = vmcore_off + phdr_ptr->p_memsz;
> +		phdr_ptr->p_offset = vmcore_off + (paddr - start);

What's paddr-start. Why following is not sufficient.

phdr_ptr->p_offset = vmcore_off

Thanks
Vivek
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