Re: [PATCH v4 7/8] vmcore: treat memory chunks referenced by PT_LOADprogram header entries in page-size boundary in vmcore_list

[HATAYAMA Daisuke]

Sorry for late reply.

(2013/04/30 4:51), Vivek Goyal wrote:
> 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.

No, there's no need to change paddr in program header. Pleaes notice 
that difference between what objects in vc_list refer to and what 
PT_LOAD program headers refer to. The former covers not only kernel 
memory but also the extra memory, while the latter the kernel memory only.

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

Yes. The extra memory is not covered by any program header, i.e. hole. 
The extra memory is not modified at all, exported with no change; if it 
is used by BIOS, users can see BIOS data there. This design comes from 
the discussion with Erick.

> > Signed-off-by: HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx>
> > ---
> >
> >   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

(paddr - start) is offset of the memory program header refers to, from 
which kernel memory starts. Pictrically:

  vmcore_off                   +----------------------+
                               |  extra memory        |
                               |  (non kernel memory) |
phdr->p_offset =               +----------------------+
  vmcore_off + (paddr - start) |                      |\
                               |  kernel memory       | phdr->p_memsz
                               |                      |/
                               +----------------------+
                               |  extra memory        |
                               |  (non kernel memory) |
  vmcore_off + size            +----------------------+

--
Thanks.
HATAYAMA, Daisuke

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