[PATCH] makedumpfile: change the wrong code to calculate bufsize_cyclic for elf dump

[bhe at redhat.com]

On 04/17/14 at 04:01am, Atsushi Kumagai wrote:
> Hello Baoquan,
> 
> >Hi Atsushi,
> >
> >I have got the test machine where bug reported and did a test. The
> >changed code can make elf dump successful.
> 
> Great, thanks for your help!
> However, I still have questions.
> 
> First, what is the difference between yours and mine?
> 
> http://lists.infradead.org/pipermail/kexec/2014-April/011535.html

Yeah, you are right, it's the same on changing the code bug. I mush
haven't read your patch carefully.

> 
> My patch includes renaming some values, but the purpose looks
> the same as yours.
> Further, you described as below, 
> 
> >On 04/14/14 at 04:02pm, Baoquan He wrote:
> >> In case elf dump, the code to calculate the cyclic buffer size is
> >> not correct. Since elf need bitmap1/2, so the needed memory for
> >> bufsize_cyclic need be double. Hence free size should be 40% of
> >> free memory, however the needed_size which free_size is compared
> >> with should be info->max_mapnr / BITPERBYTE, but not 2 times of it.
> >>
> >> Because of this, in case of free memory in 2nd kernel is not too much,
> >> OOM will happen very often.
> 
> but I still don't think this bug causes OOM.
> Even if needed_size is calculated as so much size wrongly, bufsize_cyclic
> will not exceed 40% of free memory by the check below:
> 
>     info->bufsize_cyclic = (free_size <= needed_size) ? free_size : needed_size;
> 
> So it looks that bitmap1(40%) and bitmap2(40%) will fit in 80% of free
> memory in any case.
> 
> I may misunderstand something since your patch has an effect on this
> issue in practice, could you correct me?

It definitely will cause OOM. On my test machine, it has 100G memory. So
per old code, its needed_size is 3200K*2 == 6.4M, if currently free
memory is only 15M left, the free_size will be 15M*0.4 which is 6M. So
info->bufsize_cyclic is assigned to be 6M. and only 3M is left for other
use, e.g page cache, dynamic allocation. OOM will happen.

> 
> 
> Thanks
> Atsushi Kumagai
> 
> >> Signed-off-by: Baoquan He <bhe at redhat.com>
> >> ---
> >>  makedumpfile.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/makedumpfile.c b/makedumpfile.c
> >> index 75092a8..01ec516 100644
> >> --- a/makedumpfile.c
> >> +++ b/makedumpfile.c
> >> @@ -9010,7 +9010,7 @@ calculate_cyclic_buffer_size(void) {
> >>  	 */
> >>  	if (info->flag_elf_dumpfile) {
> >>  		free_size = get_free_memory_size() * 0.4;
> >> -		needed_size = (info->max_mapnr * 2) / BITPERBYTE;
> >> +		needed_size = info->max_mapnr / BITPERBYTE;
> >>  	} else {
> >>  		free_size = get_free_memory_size() * 0.8;
> >>  		needed_size = info->max_mapnr / BITPERBYTE;
> >> --
> >> 1.8.5.3
> >>
> >>
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