[PATCH 2/2] makedumpfile: break loop after last dumpable page

[HAGIO KAZUHITO =?unknown-8bit?b?6JCp5bC+IOS4gOS7gQ==?= <k-hagio-ab@nec.com>]

-----Original Message-----
> Once the last dumpable page was processed there is no need to finish the
> loop to the last page. Thus exit early to improve performance.
> 
> Signed-off-by: Philipp Rudo <prudo@redhat.com>
> ---
>  makedumpfile.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/makedumpfile.c b/makedumpfile.c
> index 2ef3458..c944d0e 100644
> --- a/makedumpfile.c
> +++ b/makedumpfile.c
> @@ -8884,6 +8884,12 @@ write_kdump_pages_cyclic(struct cache_data *cd_header, struct cache_data *cd_pag
> 
>  	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> 
> +		/*
> +		 * All dumpable pages have been processed. No need to continue.
> +		 */
> +		if (num_dumped == info->num_dumpable)
> +			break;

This patch is likely to increase the possibility of failure to capture
/proc/kcore, although this is an unofficial functionality...

  # makedumpfile -ld31 /proc/kcore kcore.snap
  # crash vmlinux kcore.snap
  ...
  crash: page incomplete: kernel virtual address: ffff916fbeffed00  type: "pglist node_id"

In cyclic mode, makedumpfile first calculates only info->num_dumpable [1] and
frees the used bitmap, and later creates 2nd bitmap again [2] at this time.

  create_dumpfile
    create_dump_bitmap
      info->num_dumpable = get_num_dumpable_cyclic()  <<-- [1]
    writeout_dumpfile
      write_kdump_pages_and_bitmap_cyclic
        foreach cycle
          create_2nd_bitmap  <<-- [2]
          write_kdump_pages_cyclic

So with live system, num_dumped can exceed info->num_dumpable.
If it stops at info->num_dumpable, some necessary data can be missed.

Capturing /proc/kcore is still fragile and not considered enough, but
sometimes useful... when I want to capture a snapshot of memory.

(the bitmap is allocated as block, so probably it's working as some buffer?)

So I will merge the 1/2 patch, but personally would not like to merge
this patch.  How necessary is this?

Thanks,
Kazu